Ldap configurable username n uuid

This closes https://github.com/owncloud/core/issues/2151 

With this merged, the ownCloud admin will be able to do a configuration with a customized setting for internal username generation and/or UUID value. This can be used to solve or ease
- https://github.com/owncloud/apps/issues/801
- https://github.com/owncloud/core/issues/2413
- https://github.com/owncloud/core/issues/2289
- and probably also https://github.com/owncloud/core/issues/2487

Please review: @bartv2 @schiesbn @icewind1991 or others

<sub>If you wonder: yes, it requires manual configuration and cannot be done automatically due to various possibly LDAP configurations and servers. The in our POV most reliable configuration is default, but does not suit all needs.</sub>