Add a Change Owner group

Add a new system 'CHANGE_OWNER' (the patch set owner) group to allow
configuration of this group with gerrit label permissions.

To keeps it consistent with how the existing system groups
(Project Owner, Admin, etc..) are configured the Change Owner
group's logic for existing change context permissions like
Abandon, Rebase, View Drafts, etc. is untouched by this patch.
In other words an Admin can only apply this groups to a label
permission.

From upstream: https://gerrit-review.googlesource.com/#/c/48254
Bug: issue 1993
Change-Id: Ibb35b8172ce50319cfc2fca3b661fdaf7e245e5d
Merge "Add a Change Owner group" into openstack/2.8