engine: NPE in guide me dialog when exists pending approval host

When updating guide me dialog's available option buttoms. Pending
approval host's getSupportedClusterVersionsSet() returned null. The
solution here is always provide a instance of HashSet when invoking
parseSupportedVersions()

Change-Id: I890738a6319dd721e518bb8c11bc4da6769a40ce
Bug-Url: https://bugzilla.redhat.com/1091604
Signed-off-by: plysan <plysab@gmail.com>