Backport to 15.04 and update integration tests

[fgimenez]

The patch isn't very meaningful with all the renaming, these are the changes that may not be clear from the commit messages:

• The build tests were failing because of a missing "vendor" field in the package.yaml of local snaps
• The config tests failed because we were using "systemctl disable" instead of "systemctl mask" for shutting down snappy-autopilot
• updateSuite.TestUpdateToSameReleaseAndChannel was failing with "mount / is busy", none of the other tests that use the fake upload have this issue. It's been skipped until we understand the problem.

Also, the jenkins job has been updated to take into account the target branch, when it is "15.04" the tests are run in a 15.04/edge image

[mvo5]

This is very very nice indeed! A bit hard to review as its so much, but given that its a backport so pretty much everything was reviewed before I think its fine. 👍