<issue_start><issue_comment>Title: Rename the `mongo` interface to be more specific
username_0: See, e.g., `mongodb-datasource` or something that expresses "I want to QUERY mongo" while leaving naming space for other relations powered by mongo, like a hypothetical "mongo-administration" if someone goes and charms Mongo OpsManager.
<issue_comment>username_1: The MongoDB charm has changed the name to `mongodb_datastore` (note underscore) as the relation interface name for its consumer charms. In view of this I am closing this ticket.<issue_closed>