Create "Push Masters Admin" group

The current flow for branching qt requires gerrit administrator privileges to add oneself to the "Push Masters" group as part of the branching script. These permissions are intended to be temporary, since the rights given by the Push Masters group are rather dangerous.

To remove the need for gerrit admins to run the script, a "Push Masters Admin" group should be created. This group has no explicit permissions and grants no new rights other than to allow members of that group to add and remove themselves from the Push Masters group, which is an automatic part of the branching script.

Add ankokko , aksalova, and janihe to this group.