Ldap plugin implementation currently supports only 'simple' authentication method

### Other authentication methods:
- GSSAPI
- KRB5 etc..

[Example link](http://docs.oracle.com/javase/jndi/tutorial/ldap/security/gssapi.html)

Start by locating the plugin's LDAP authentication implementation and determine how it currently restricts authentication to the simple method. Review the linked Oracle GSSAPI example and the listed GSSAPI and KRB5 methods. Done means the plugin supports the agreed additional authentication methods with suitable coverage, but the issue does not define that scope.

Written by the indexing model from the issue text.