Let test plans specify clouds

[kwmonroe]

It would be nice to say "this test plan should run on aws and gce". We know, for example, that bigtop big data bundles do not work on joyent, so we would like to blacklist that cloud at a test-plan level.

If not specified, cwr should assume all clouds.

[johnsca]

There is some overlap here with juju-solutions/bundletester#48