Make it possible to force case on identifier when using an LDAP directory

Sometimes LDAP directory used for users is case-insensitive => when user login case changes, this should not have any repercussion on Nuxeo.

The easiest way to implement it is to use the same case on Nuxeo side, whatever information is actually stored in the LDAP directory => make it possible to force upper or lower case by configuration on the LDAP descriptor.

Note that references (or inverse references) to this directory should not hold any additional configuration: we assume that in this case the LDAP directory is case-insensitive so the referenced ids should be found whatever the case used.