Add admin_groups

[flashpixx]

Proposed change

Add a config item admin_groups as a list to define LDAP groups for administration, similar issues are defined in #72 or #160

Who would use this feature?

In a complex infrastructure there are mostly not single admins, so mostly there is a group of admins which is defined by a LDAP group entry

Suggest a solution

Add equal to the allowed_groups the item admin_groups and match it to the internal admin behaviour. On the login the scope can be valid, so that if a user is memberof one of these group administrator access is granted.