Add SSH command to set group members

Add a new SSH command that allows to
- add users to groups
- remove users from groups
- include groups into groups
- exclude groups from groups

That change is based on previous work of Edwin Kempin:
https://gerrit-review.googlesource.com/#/c/30009

Bug: issue 1095
Change-Id: I9a80bc52f5a9847d4e8b4d848149167844eadd19
5 files changed