According to the Ubuntu 12.04 release notes, up until Ubuntu 11.10
admin access was granted via the "admin" unix group, but was changed
to the "sudo" group to be more consistent with Debian et al.
Remove the now unnecessary group
Modify the install-ansible role to set some directory ownership to
root:root; there didn't seem to be any reason to use admin here.
This means the "users" role is no longer required in the bridge.yaml,
as it is run from the base playbook anyway.
Change-Id: I6a7fdd460fb472f0d3468eb080aebbb010931e11
114953cbff