Create run-cover role

We incorrectly used the run-tox role for our coverage jobs.  Correct
this by creating the run-cover role and updating tox-cover job to use
it.

Change-Id: I775fa00577856c3914266d23d32e9333b70e7db7
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/playbooks/tox-cover.yaml b/playbooks/tox-cover.yaml
index 942aa23..ca391e1 100644
--- a/playbooks/tox-cover.yaml
+++ b/playbooks/tox-cover.yaml
@@ -1,5 +1,3 @@
 - hosts: all
-  vars:
-    run_tox_envlist: cover
   roles:
-    - run-tox
+    - run-cover