Add default-branch property to projects

Some projects don't have a 'master' branch.  Allow users to specify
the default-branch of a project so the executor can know what branch
to fall back to when preparing repos for a test.

Pass that information to the executor.  A subsequent change will
begin to use it.

Change-Id: I20723ac0d3edc9eef38445fb5b61ba3122fe58bb
3 files changed