Fix barbican service_available check

Tempest plugins should not explicitly set option from other plugins. The
change I7013888f94261d94e1cd4c3167dc84da7125d1da set
service_available.barbican to false if the barbican plugin doesn't
exist, but cause duplicate error if both are installed. The right way to
check if a service is available or not is checking if the attribute
exists and set the default to false. This patch fix the duplicate error.

Depends-On: https://review.openstack.org/#/c/639153/
Change-Id: I1ba353328e759391cc6a46d95b74c85c4cea6d92
Closes-Bug: 1817154
3 files changed