We switched the tripleo-container-image-prepare script in
tripleo-common, but a user will likely run the 'openstack tripleo
container image prepare' command. Currently it uses the default which is
the threading executor.
Poke tripleo common lower/requirements as well.
Change-Id: Ifc5b46633a1f9fc9378eaa17170f6664d566c3c4
Related-Bug: #1844446