Validate cpi-test stemcell generation
Validate cpi-test stemcell generation

'vmbuilder' generates bogus stemcells once in a while.

This change detects if the stemcell is bad by looking at vmware-tool status.
If the generated stemcell is invalid the test automatically generates a new
one.

Change-Id: I37d50288a03759bbba49b593a21ce24b8a9e8032

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh