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

@@ -14,6 +14,8 @@
 
 TAG=$1
 
+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"
@@ -25,3 +27,4 @@ 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