Switch to new grenade job name

The integrated gate template (integrated-gate-py3) has been switched
to the new grenade name (grenade-py3 -> grenade). This repo uses the
template but also has 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

Depends-On: https://review.opendev.org/725148
Change-Id: I73e67c0e01ca231678903b2460dab672f17251e3

.zuul.yaml

@@ -627,7 +627,7 @@
         - swift-dsvm-functional-py3:
             voting: false
             irrelevant-files: *dsvm-irrelevant-files
-        - grenade-py3:
+        - grenade:
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
@@ -676,7 +676,7 @@
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
-        - grenade-py3:
+        - grenade:
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$