Skip tests when heat is unavailable

Currently the [service_available] heat parameter is not actually used
and the plugin tests are always executed regardless of availability
of Heat service.

This change ensures that the parameter is honored so that users can
disable plugin tests even if the plugin package is enabled.

Change-Id: I148d7a9c92f210d4584798131a1b23f648f6e5e3
3 files changed