Update docs to reference #openstack-keystone

Our documentation was still referencing #openstack as the place to find
Keystone folks. While that is still true, we need to update the docs so
that we inform people about our #openstack-keystone channel.

Change-Id: I8035fd08d5ef74f698f793d6b90ce1547a26aa56
Partial-Bug: #1283943
Merge "Update docs to reference #openstack-keystone"