Feature #813 Usergroups can be linked to an LDAP auth_source

This pull request lets usergroups be linked to an LDAP authentication source. This allows an organization to have foreman usergroups match its LDAP user groups, so that users that belong to LDAP user groups can be automatically matched to their equivalent usergroups in Foreman. 

When you create a new usergroup and you want it to be linked to an LDAP source, It performs a check to make sure the usergroup exists in LDAP first. If it does not exist, it will not let the user create it. 

Since LDAP information can change, there is a rake task attached that updates the foreman usergroup with this information.

http://projects.theforeman.org/issues/813
It's not exactly the class checking, but more that we are checking the class and have LDAP specific code outside of the LDAP class.

How about this?  https://github.com/domcleal/foreman/commit/f4d755d23c242137774fe8e5f2cd4a502df5ed7d.  The user logic stays here, but LDAP logic is in the auth source. Please feel free to squash in if you like it, but I'd appreciate you testing it, I only checked unit tests.

(I also fixed a potential issue with update_usergroups being called on non-LDAP auth sources like external.)