[designate] Add coordination backend for designate workers

Add coordination backend configuration to designate.conf which is
required in multinode environments. Fixes warning from designate:

WARNING designate.coordination [-] No coordination backend configured,
assuming we are the only worker. Please configure a coordination backend

Change-Id: I23c4d2de7e3f9368795c423000a4f9a6c3a431e2
Closes-Bug: #1843842
Related-Bug: #1840070