Fix MongoDB template

Using:

• nuxeo-server-9.3-I20171109_2314-tomcat
• windows 10
• edge
• redis
• mongodb 3.4.10

• Follow the wizard and choose mongodb as db backend
• Select webui only as optional addons in wizard
• Finish wizard and expect nuxeo to restart

=> nuxeo stops, does not restart
=> logs shows an error in the mongodb template, see file attached

The following has evaluated to null or missing:
==> nuxeo.db.driver  [in template "datasources-config.xml.nxftl" at line 7, column 64]

expected: server should restart and no message should be in the logs