Rename openstack-admins to openstack-planet-core.

* .../gerrit/acls/openstack/openstack-planet.config: The
openstack-admins Gerrit group is only used by the openstack-planet
ACL any longer, so renamed to openstack-planet-core accordingly.

Change-Id: Ie1a5714b71d9e47a3b31b9fede7dac9347b65490
Reviewed-on: https://review.openstack.org/34452
Reviewed-by: matthew wagoner <zxkuqyb@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
Release Managers replaces openstack-release.

* doc/source/gerrit.rst: Update documentation to indicate that the
Release Managers group in Gerrit has supplanted the old
openstack-release group.

Change-Id: Id4ac08583ec3723289a1ebffbd1bbed1e591b760
Reviewed-on: https://review.openstack.org/34462
Reviewed-by: Elizabeth Krumbach Joseph <lyz@princessleia.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins