Fix MultiDirectory with LDAP

1. install nuxeo 10.10
2. install web-ui and jsf
3. configure multidirectory using multiUserGroup (full multi)
4. login
5. in the JSF admin interface try to list the users or the groups, an 500 error pops up
6. in the web ui interface try to list the users and the groups, only the groups are listed
7. observe the following stack trace in the logs:

8. 2019-02-14T10:44:53,147 ERROR [WebEngineExceptionMapper] org.nuxeo.ecm.directory.DirectoryException: getEntry failed: Need to specify class name in environment or system property, or as an applet parameter, or in an application resource file:  java.naming.factory.initial
   org.nuxeo.ecm.directory.DirectoryException: getEntry failed: Need to specify class name in environment or system property, or as an applet parameter, or in an application resource file:  java.naming.factory.initial
   	at org.nuxeo.ecm.directory.ldap.LDAPSession.getEntryFromSource(LDAPSession.java:434) ~[nuxeo-platform-directory-ldap-10.10.jar:?]
   	at org.nuxeo.ecm.directory.DirectoryCache.getEntry(DirectoryCache.java:121) ~[nuxeo-platform-directory-api-10.10.jar:?]
   	at org.nuxeo.ecm.directory.BaseSession.getEntry(BaseSession.java:344) ~[nuxeo-platform-directory-api-10.10.jar:?]
   	at org.nuxeo.ecm.directory.multi.MultiDirectorySession.getEntry(MultiDirectorySession.java:353) ~[nuxeo-platform-directory-multi-10.10.jar:?]
   	at org.nuxeo.ecm.directory.multi.MultiDirectorySession.query(MultiDirectorySession.java:785) ~[nuxeo-platform-directory-multi-10.10.jar:?]
   	at org.nuxeo.ecm.platform.usermanager.UserManagerImpl.searchUsers(UserManagerImpl.java:1108) ~[nuxeo-platform-usermanager-core-10.10.jar:?]
   	at org.nuxeo.ecm.platform.usermanager.UserManagerImpl.searchUsers(UserManagerImpl.java:1096) ~[nuxeo-platform-usermanager-core-10.10.jar:?]
   	at org.nuxeo.ecm.platform.usermanager.UserManagerImpl.searchUsers(UserManagerImpl.java:1004) ~[nuxeo-platform-usermanager-core-10.10.jar:?]
   	at org.nuxeo.ecm.platform.usermanager.providers.AbstractUsersPageProvider.searchAllUsers(AbstractUsersPageProvider.java:131) ~[nuxeo-platform-usermanager-api-10.10.jar:?]
   	at org.nuxeo.ecm.platform.usermanager.providers.AbstractUsersPageProvider.searchUsers(AbstractUsersPageProvider.java:138) ~[nuxeo-platform-usermanager-api-10.10.jar:?]
   	at org.nuxeo.ecm.platform.usermanager.providers.AbstractUsersPageProvider.computeCurrentPage(AbstractUsersPageProvider.java:80) ~[nuxeo-platform-usermanager-api-10.10.jar:?]
   	at org.nuxeo.ecm.platform.usermanager.providers.NuxeoPrincipalsPageProvider.getCurrentPage(NuxeoPrincipalsPageProvider.java:46) ~[nuxeo-platform-usermanager-api-10.10.jar:?]
   	at org.nuxeo.ecm.automation.core.util.PaginablePageProvider.<init>(PaginablePageProvider.java:43) ~[nuxeo-core-io-10.10.jar:?]
   	at org.nuxeo.ecm.restapi.server.jaxrs.PaginableObject.getPaginableEntries(PaginableObject.java:90) ~[nuxeo-rest-api-server-10.10.jar:?]
   	at org.nuxeo.ecm.restapi.server.jaxrs.PaginableObject.getPaginableEntries(PaginableObject.java:85) ~[nuxeo-rest-api-server-10.10.jar:?]
   	at org.nuxeo.ecm.restapi.server.jaxrs.usermanager.AbstractUMRootObject.search(AbstractUMRootObject.java:75) ~[nuxeo-rest-api-server-10.10.jar:?]
   	...
   Caused by: javax.naming.NoInitialContextException: Need to specify class name in environment or system property, or as an applet parameter, or in an application resource file:  java.naming.factory.initial
   	at javax.naming.spi.NamingManager.getInitialContext(NamingManager.java:662) ~[?:1.8.0_181]
   	at javax.naming.InitialContext.getDefaultInitCtx(InitialContext.java:313) ~[?:1.8.0_181]
   	at javax.naming.InitialContext.getURLOrDefaultInitCtx(InitialContext.java:350) ~[?:1.8.0_181]
   	at javax.naming.directory.InitialDirContext.getURLOrDefaultInitDirCtx(InitialDirContext.java:106) ~[?:1.8.0_181]
   	at javax.naming.directory.InitialDirContext.search(InitialDirContext.java:286) ~[?:1.8.0_181]
   	at sun.reflect.GeneratedMethodAccessor348.invoke(Unknown Source) ~[?:?]
   	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_181]
   	at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_181]
   	at org.nuxeo.ecm.directory.ldap.LdapRetryHandler.invoke(LdapRetryHandler.java:58) ~[nuxeo-platform-directory-ldap-10.10.jar:?]
   	at com.sun.proxy.$Proxy165.search(Unknown Source) ~[?:?]
   	at org.nuxeo.ecm.directory.ldap.LDAPSession.getLdapEntry(LDAPSession.java:365) ~[nuxeo-platform-directory-ldap-10.10.jar:?]
   	at org.nuxeo.ecm.directory.ldap.LDAPSession.getEntryFromSource(LDAPSession.java:428) ~[nuxeo-platform-directory-ldap-10.10.jar:?]
   	... 134 more
   

Expected behavior: there should be no error
Note: the same configuration works fine with 9.10
Note 2: a full LDAP configuration works fine in 10.10 too, so this seems to be specific to the multidirectory mode