Update tripleo-common to 12.6.0
Update tripleo-common to 12.6.0

Remove tripleo-container-image-prepare since removed in
https://github.com/openstack/tripleo-common/commit/ecf838cba62e1996b4777e6d35e146b8ed0787e2

Change-Id: I4dd117dc94850e619e86aa4fbd9ed72771069a5b

openstack/rpm-packaging,openstack/rpm-packaging