Limit overcloud-extras playbook runs to overcloud nodes

If a deployer modifies Kayobe playbooks included by overcloud-extras
(for example, docker-registry) to run on non-overcloud hosts (for
example the seed), `kayobe overcloud` commands running overcloud-extras
will try to run on those non-overcloud hosts as well.

Change-Id: I13dde285c3f873fdc53b90706352a9544e90a62a
Story: 2004819
Task: 28985