Replace opensatck-terracotta with openstack-nimble in gerritbot channels

As we are at our limits of channels we can serve with one gerritbot client,
We would like to replace openstack-terracotta with openstack-nimble here to
make Nimble project get notifications from gerrit. This has been approved
by Terracotta PTL.

Change-Id: Id161ccd4068f519b5d3934a09249e4d8c80f1ca6

gerritbot/channels.yaml

@@ -1436,6 +1436,16 @@ openstack-nfv:
   branches:
     - master
 
+openstack-nimble:
+  events:
+    - patchset-created
+    - change-merged
+    - x-vrif-minus-2
+  projects:
+    - openstack/nimble
+  branches:
+    - master
+
 openstack-nova:
   events:
     - patchset-created
@@ -2211,16 +2221,6 @@ openstack-telemetry:
     - stable/mitaka
     - stable/liberty
 
-openstack-terracotta:
-  events:
-    - patchset-created
-    - change-merged
-    - x-vrif-minus-2
-  projects:
-    - openstack/terracotta
-  branches:
-    - master
-
 openstack-third-party-ci:
   events:
     - patchset-created