Move the milestone-proposed logic to tag script

The release pipeline does not have any branch information, because tag
events are branchless. So, in the script, check to see if the tag is in
milestone-proposed, and if so, submit the merge commit.

Change-Id: Ia46bf6ad7826571d39884cf7cd8165c5cea678e6

modules/jenkins/files/slave_scripts/merge_tags.sh

@@ -14,14 +14,17 @@
 
 TAG=$1
 
-git config user.name "OpenStack Jenkins"
-git config user.email "jenkins@openstack.org"
-git config gitreview.username "jenkins"
+if $(git tag --contains origin/milestone-proposed | grep "^$TAG$" >/dev/null)
+then
+    git config user.name "OpenStack Jenkins"
+    git config user.email "jenkins@openstack.org"
+    git config gitreview.username "jenkins"
 
-git review -s
-git checkout master
-git reset --hard origin/master
-git merge -s ours $TAG
-# Get a Change-Id
-GIT_EDITOR=true git commit --amend
-git review -R -t merge/release-tag
+    git review -s
+    git checkout master
+    git reset --hard origin/master
+    git merge -s ours $TAG
+    # Get a Change-Id
+    GIT_EDITOR=true git commit --amend
+    git review -R -t merge/release-tag
+fi

modules/openstack_project/files/zuul/layout.yaml

@@ -271,8 +271,6 @@ jobs:
     branch: ^(?!stable/(folsom|grizzly)).*$
   - name: manuals-upstream-translation-update
     branch: ^master$
-  - name: ^.*-merge-release-tags$
-    branch: milestone-proposed
   - name: gate-hacking-python33
     voting: false
   - name: gate-hacking-pypy