CORE: save up 1 DB query in GroupsManager.addMember()

- We can check authz in method on Group for VOADMIN, so no need to
  call getVo(sess, Group). Also check for matching VO can be done
  on Group's and Member's voIds.
Merge pull request #224 from zlamalp/simplify

CORE: save up 1 DB query in GroupsManager.addMember()