add an option to set custom list of admin groups

This commit enables to optionally set a single or a list of admin groups (as opposed to the current hardcoded "admin"). It falls back to "admin" if list is empty.
This is very useful in an environment where the "admin" group is already otherwise in use, there already exists a differently named group ("Administrators" or whatnot) or a group of users should be able to administrate owncloud but not other services ("owncloud_admins"). Especially large ldap environments will have this requirement.