Check if heat machine id file exists before cat'ing it

It might not exist if we don't have a heat test image.
Change order so that tempest appears and runs after all barclamps

It was still before the trove barclamp, and the chef recipes were run
too early too.
Merge pull request #103 from vuntz/order

Change order so that tempest appears and runs after all barclamps
Merge pull request #102 from vuntz/cat-non-existing-file

Check if heat machine id file exists before cat'ing it
Adapt code for Cinder Multi backend support
Enable multi-backend testing in cinder whenever possible
Merge pull request #101 from dirkmueller/multi_backend

Adapt code for Cinder Multi backend support