only run branchless jobs on master

tempest actually still has old branches, so we need to make sure
that we only run the branchless jobs on master and not on old
stable branches.

Closes-Bug: #1310368

Change-Id: I99e6c2185b40eb0ae32b56d18d0f7dad3e177f2a
Merge "only run branchless jobs on master"