lxc sandbox - improved CI with sandboxed source tests

Addresses part 1 of  https://bugs.launchpad.net/oq-engine/+bug/1177319

With this improvement we decouple source tests from host machine and allow concurrent builds of different branches.

The only thing that remain on the host is the build of packages but using different workspace it is race proof.

It would be the building block for further automations  but already now you can use it to test your own branches when you want.

HOWTO: to test different branches you can create a copy of the job and reconfigure it to follow your repo/branch (enabling or disabling polling, night build, etc.)
dependencies from our libraries are solved in this way:
  - search them in the same repository  with the same branch name
  - if not found: search them in the gem repository  with the same branch name
  - if not found  search them in the gem repository using master branch name