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
diff --git a/stackrc b/stackrc
index 10117f2..4b04941 100644
--- a/stackrc
+++ b/stackrc
@@ -499,7 +499,7 @@
 
 # pbr drives the setuptools configs
 GITREPO["pbr"]=${PBR_REPO:-${GIT_BASE}/openstack/pbr.git}
-GITBRANCH["pbr"]=${PBR_BRANCH:-$TARGET_BRANCH}
+GITBRANCH["pbr"]=${PBR_BRANCH:-$BRANCHLESS_TARGET_BRANCH}
 
 
 ##################
@@ -554,7 +554,7 @@
 
 # diskimage-builder tool
 GITREPO["diskimage-builder"]=${DIB_REPO:-${GIT_BASE}/openstack/diskimage-builder.git}
-GITBRANCH["diskimage-builder"]=${DIB_BRANCH:-$TARGET_BRANCH}
+GITBRANCH["diskimage-builder"]=${DIB_BRANCH:-$BRANCHLESS_TARGET_BRANCH}
 GITDIR["diskimage-builder"]=$DEST/diskimage-builder
 
 # neutron-lib library containing neutron stable non-REST interfaces