Use lhcb-master instead of lhcb-master-ref as reference for ci-test

Currently, building the lhcb-master-ref slots can take a lot of resources. We can probably use the latest lhcb-master build as reference without losing much. The lhcb-master-mr CouchDB document should reflect that accordingly.

It's desirable, if not too hard, to keep the ability to also launch a new lhcb-master-ref slot with /ci-test (for the "branch" mode, as opposed to --merge).

/cc @chasse