[useradmin] Ensure posix groups and other groups are separated

If you specify "memberUid" as the "group_member_attr" and try to add sync groups and import new members while using posixGroups in ldap, it fails with:

Processing exception: Distinguished Name provided does not contain configured Base DN. Base DN: dc=test,dc=com, DN: hadoop: None

This is because of this section of ldap_access.py:

member_attr = desktop.conf.LDAP.GROUPS.GROUP_MEMBER_ATTR.get()
if member_attr in data:
ldap_info['members'] = data[member_attr]
else:
ldap_info['members'] = []

if 'posixGroup' in data['objectClass'] and 'memberUid' in data:
ldap_info['posix_members'] = data['memberUid']
else:
ldap_info['posix_members'] = []

This adds the users to both members and posix_members because they specified the memberUid and we only include memberUid in the posixGroup if statement. It works if you don't specify group_member_attr and the default of "member" is used, however, in posixGroup environments, they won't necessarily leave "group_member_attr" empty, they would probably set it to memberUid.