Find LDAP user by attribute using a bind-account. Requires additional…

[jayjensen]

LDAP auth:
Added the possibility to login by an attribute search (e.g.: uid).
Requires:

1. Additional config settings
2. A bind-user and the bind password as well as the permissions to
   this user (on the LDAP server) to request user information.
3. The searched attribute (e.g. the "uid" attribute).

[jayjensen]

Sorry, the line:

print('Gruppen', new_groups_content)

should not be there.