browsertests runner

I have split the browsertest run in two jobs:

- qa-browsertests-build : install bundler and gems, snapshot the result
- qa-browsertests-run : reuses the snapshot and actually run tests

The idea is to only do the gems setup once per merge and reuse the
resulting install whenever we have to run the tests.

It is awesome how productive I feel between 1am and 5am (bug 20281)

bug: 54388
Change-Id: I7f5a428e2524545743cd88616b85871d1e073c6d