[useradmin] Add support for POSIX LDAP Groups

Some LDAP implementations are strictly used to provide POSIX groups to the OS. As a result, they may not have the DN of a user to specify members. See ldapsearch output below:

dn: cn=posixgroup1,ou=People,dc=test,dc=com
objectClass: groupOfUniqueNames
objectClass: posixGroup
objectClass: top
gidNumber: 5000
memberUid: user.0
cn: posixgroup1

In this case, the group attribute is memberUid and it points to the user's uid attribute instead of DN. As a result, in an environment that only has posix groups, Hue can't import groups.