Minor nessy fixups

This PR supersedes #352.

Here is a recap of the rollout strategy:

1. (done) Deploy code that can use Nessy (as a non-mandatory lock) when configuration directs it to.
2. (done) Wait until no snapshots that cannot accept that configuration are active.
3. Set the Nessy configuration in genome.sh & monitor for problems.
4. Update configuration to make Nessy locks mandatory.
5. Wait until all active snapshots have mandatory Nessy locking in place.
6. Update code to turn off NFS-based locks.

This PR makes minor fixes so that we can implement item (3) above.  Once this is merged, we can enable `GENOME_NESSY_SERVER` in jenkins,  and if satisfied make production configuration changes.

As briefly discussed in #352, both CLE and LIMS snapshots need to be considered for step (5).
Maybe that's why I made `add_backend()`? ;-)