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
I'm unsure how to handle this, what worries you is the class checking (admittedly not too elegant, I can check auth_source.type...)? If I move the logic to auth_source , I'll have to call ```external_usergroup.usergroup.users```
```external_usergroup.usergroup.external_usergroups``` and so on, plus I'll still have to keep checking the kind of auth source because external usergroups refresh might not always be linked to LDAP.