Validate instances are accessible before messing with their disks

This should help to isolate failures that are generic from specific test
failures related to barbican. If a test can't pass simple connectivity
check, it's probably broken, and there is no point to attach disks,
because it won't work.

Related-Bug: #1742936
Change-Id: I96cd35392f638c0e5b668f4f3b15cf4cd6114b71
2 files changed