Use Tempest slow job to run all slow tests

tempest-slow job run all the slow tests including API
or scenario slow tests. By running this job will help to
avoid any regressions across projects.

This new job "tempest-slow" will run as voting on check pipeline
and same job run on nova and neutron gate.

Tempest also run "tempest-slow" as voting job.

Detailed discussion on ML[2].

[1] http://git.openstack.org/cgit/openstack/tempest/tree/.zuul.yaml#n147
[2] http://lists.openstack.org/pipermail/openstack-dev/2018-May/130394.html

Change-Id: Ifb982a464fdd8313c1172b9ada674c436d73fecc

.zuul.yaml

@@ -147,6 +147,21 @@
               - ^setup.cfg$
               - ^tools/.*$
               - ^tox.ini$
+        - tempest-slow:
+            irrelevant-files:
+              - ^(test-|)requirements.txt$
+              - ^.*\.rst$
+              - ^api-ref/.*$
+              - ^cinder/hacking/.*$
+              - ^cinder/locale/.*$
+              - ^cinder/tests/functional.*$
+              - ^cinder/tests/unit.*$
+              - ^contrib/block-box.*$
+              - ^doc/.*$
+              - ^releasenotes/.*$
+              - ^setup.cfg$
+              - ^tools/.*$
+              - ^tox.ini$
     gate:
       jobs:
         - cinder-tox-compliance