Avoid duplicate 'barbican' service_available option

The cinder and barbican tempest plugins both need the 'barbican'
service_available option to be defined, but they both can't register
the option. This patch ensures the cinder plugin registers the option
only when the barbican plugin isn't present.

Closes-Bug: #1991068
Change-Id: I51a22afae4fc98e2c2b8c1e82e8211a27649022c
2 files changed