Changes

jobs/master-builds: add param BRANCH

Sometimes it is desirable to run master jobs from a different branch:
* As discussed with Philipp: running pysim CI on a branch with many
  patches once before submitting to gerrit (it takes much longer in
  gerrit to run on each individual patch)
* When adding/modifying master jobs to test if they still work as
  expected with additional patches in a branch.

It is undesirable to have mail notifications if not running on master,
the next patch will make it easy to disable those.

Change-Id: I5fb64024dc4ad70cb3dd019bd505aacc58d14380