shared-db relation should probably set database in relation settings

[Robert Ayres]

shared-db relation takes a host, database and user and then sets database host and password. If you use this interface to relate to multiple databases (by firing '*-relation-changed' event with a new database) then there is a chance (highly unlikely :) ) that the password generated matches a previously fired event and if the new relation settings aren't different, then the return event won't fire.

A simple way to fix this would be to set the database requested along with the database host and password, which should always give a unique combination. Or alternatively provide a new relation setting that allows multiple databases to be specified up front.