Switch to new grenade job name

The integrated gate templates has switched to the new grenade name
(grenade-py3 -> grenade). This repo uses an integrated gate template
but also listed the job for irrelevant files an extra entry.

Rename the job following the template change to avoid duplicate
grenade runs.

Details:
- http://lists.openstack.org/pipermail/openstack-discuss/2020-May/014602.html

Change-Id: I4afc73301965b582fd338012f0ddc60f9b85fbb0

.zuul.yaml

@@ -644,7 +644,7 @@
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/.*$
               - ^(.gitreview|.mailmap|AUTHORS|CHANGELOG)$
-        - grenade-py3:
+        - grenade:
             irrelevant-files:
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/.*$
@@ -712,7 +712,7 @@
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/.*$
               - ^(.gitreview|.mailmap|AUTHORS|CHANGELOG)$
-        - grenade-py3:
+        - grenade:
             irrelevant-files:
               - ^(api-ref|doc|releasenotes)/.*$
               - ^test/.*$