Restored testContext to prior behavior, with exception that cached
credentials are nulled on failure.

modified getGoogleCompute NOT to call testContext()


NOTE: if ctx.getRegionId() != null, the connection is not tested, and
the credential will not be invalidated.  I assume(hope) that this is a
test for having already been through this method to avoid excessive api
calls.
Merge pull request #173 from unwin/2014.08-FB-5217-5201

Please merge and try for a release candidate.