Allow requesting a mariadb set-up to replicate another mariadb:
- bootstrap-url: bootstrap from a statically served backup file
- primary-url: replicate from a primary mariadb

This happens in mariadb first initialization, when no data exists yet.
That way existing data in a non-replicating mariadb cannot be deleted
by setting the replication parameters after the fact.

A promise checks that the state (replica / primary, replication source)
of the running mariadb matches the requested state; but if it doesn't,
the mariadb will not automatically converge without human intervention
if ~/srv/mariadb data directory already exists, to avoid deleting data.