Make gate-trove-python34-db voting; add gate job

As part of the Trove blueprint, the python34-db check job must become
voting to avoid Python 3 regressions.

Also use the python3-db-jobs template to add gate-trove-python34-db
to gate jobs.

Partially implements: blueprint trove-python3
Change-Id: If960ecd567e4dc4eba559e35b767fbbf0d02e7cd

zuul/layout.yaml

@@ -2213,7 +2213,6 @@ jobs:
 
   - name: gate-trove-python34-db
     branch: ^(?!stable/(?:juno|kilo|liberty|mitaka)).*$
-    voting: false
 
   - name: gate-anchor-tox-bandit
     branch: master
@@ -11469,6 +11468,7 @@ projects:
     template:
       - name: merge-check
       - name: python-db-jobs
+      - name: python3-db-jobs
       - name: openstack-server-publish-jobs
       - name: openstack-server-release-jobs
       - name: periodic-db-kilo
@@ -11478,7 +11478,6 @@ projects:
       - name: translation-jobs
       - name: release-notes-jobs
     check:
-      - gate-trove-python34-db
       - gate-trove-tox-checklinks
       - gate-trove-tox-doc-publish-checkbuild
       - gate-trove-functional-dsvm-mysql