Add tempest-slow job to run the tempest slow tests

By default, tempest-full jobs don't run the "slow" tests
which test useful things like encrypted volumes, swap
volume, etc. The slow tests aren't part of the default
tempest run since they can be, well, slow.

Tempest has tempest-slow job now which run all the slow
tagged tests.[1]

Detailed discussion on ML[2].

[1] http://git.openstack.org/cgit/openstack/tempest/tree/.zuul.yaml#n146
[2] http://lists.openstack.org/pipermail/openstack-dev/2018-May/130394.html

Conflicts:
      .zuul.yaml

NOTE(mriedem): The conflict is due to not having change
I51adbbdf13711e463b4d25c2ffd4a3123cd65675 nor change
I24a5f73c29094a23e2fdef8ee8b43601300af593 in Pike.

Related-Bug: #1783405

Change-Id: I5cb829beebcc59de682cba927f5b01793ec70d00
(cherry picked from commit f6390f71ff)
(cherry picked from commit 105dbbcbf2)

.zuul.yaml

@@ -81,6 +81,21 @@
     check:
       jobs:
         - nova-live-migration
+        - tempest-slow:
+            irrelevant-files:
+              - ^(placement-)?api-.*$
+              - ^(test-|)requirements.txt$
+              - ^.*\.rst$
+              - ^.git.*$
+              - ^doc/.*$
+              - ^nova/hacking/.*$
+              - ^nova/locale/.*$
+              - ^nova/tests/.*$
+              - ^releasenotes/.*$
+              - ^setup.cfg$
+              - ^tests-py3.txt$
+              - ^tools/.*$
+              - ^tox.ini$
         - nova-tox-functional
         - nova-tox-functional-py35
         - neutron-grenade-multinode:
@@ -198,6 +213,21 @@
     gate:
       jobs:
         - nova-live-migration
+        - tempest-slow:
+            irrelevant-files:
+              - ^(placement-)?api-.*$
+              - ^(test-|)requirements.txt$
+              - ^.*\.rst$
+              - ^.git.*$
+              - ^doc/.*$
+              - ^nova/hacking/.*$
+              - ^nova/locale/.*$
+              - ^nova/tests/.*$
+              - ^releasenotes/.*$
+              - ^setup.cfg$
+              - ^tests-py3.txt$
+              - ^tools/.*$
+              - ^tox.ini$
         - nova-tox-functional
         - nova-tox-functional-py35
         - tempest-full: