Fix target branch for pbr and diskimage-builder

The target branch was centralized in change
I82aa19e739eeda3721bac1cb5153ad0bf2d1125a but there
are two issues, pbr and diskimage-builder are using
TARGET_BRANCH which gets changed to stable/* for
each openstack stable branch that gets created for
devstack, e.g. I861068ae1a9902cef61c52c70dda7bb42f4371a0,
but pbr and diskimage-builder don't have stable branches
so they should be using BRANCHLESS_TARGET_BRANCH i.e. master.

Change-Id: I47ac7a7e194ca6d613d0ccaebfd557346644c2df
1 file changed