*-relation-broken has no way to identify which remote service is being broken

While working on the mysql formula, I needed a way to record the fact that a relation had been broken, so that when it was re-added, the code path would be slightly different since the database already existed.

Unfortunately, the environment doesn't seem to contain an indicator as to which relationship is broken. ENSEMBLE_RELATION only tells me which of my relations are being broken, but not what remote service has been unrelated.

The workaround is to run 'relation-list' which seems to somehow know which relationship is being broken and only return the members of it. Whatever it uses to know that, should be exposed to the hook in the environment.