Replace opensatck-terracotta with openstack-nimble in gerritbot channels
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

openstack-infra/project-config,Tesora/tesora-project-config,openstack-infra/project-config,Tesora/tesora-project-config