Switch to new grenade job name

The integrated gate templates has switched to the new grenade name
(grenade-py3 -> grenade).

Rename the job here as well so that grenade-py3 is not used on master
and stable/ussuri anymore.

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

Change-Id: I05a312fb3f2cb44baeecd4b0609f71a21b1c536d

.zuul.d/project.yaml

@@ -43,7 +43,7 @@
             irrelevant-files:
               - ^(test-|)requirements.txt$
               - ^setup.cfg$
-        - grenade-py3:
+        - grenade:
             irrelevant-files:
               - ^(test-|)requirements.txt$
               - ^setup.cfg$
@@ -87,7 +87,7 @@
             irrelevant-files:
               - ^(test-|)requirements.txt$
               - ^setup.cfg$
-        - grenade-py3:
+        - grenade:
             irrelevant-files:
               - ^(test-|)requirements.txt$
               - ^setup.cfg$