mongo-memory-profile not set when enable-ha used

[Tim Penhey]

When a controller is bootstrapped with --config mongo-memory-profile=default, the initial machine has `mongomemoryprofile: default` in the agent.conf and all is good.

If you then `enable-ha` the new machines have no `mongomemoryprofile` line written to the agent.conf file, and the juju-db.service file acts as if the profile is set to "low".