Limit backups to dumping certain databases.

This patch takes an opt-out approach, explicitly ignoring the backups and presence databases.  This requires some trickiness with mongodump: --oplog isn't supported with the --database option.  We are going with option 2 on https://lists.ubuntu.com/archives/juju-dev/2014-October/003818.html.  However, we are not using the --oplog option.  That will be addressed in a subsequent patch.