Improve handling of unqualified Tango DB hostname

If you specify your Tango database address as a bare hostname, your attributes will end up stored with the database's full, canonical name. However, when you go to re-apply the file, these attributes will be interpreted as having changed, because yaml2archiving compares a device address constructed from the provided bare hostname to a device address that uses the full canonical name.

This change uses the canonical name of the db (via get_db_host()) when generating the set of desired attributes, instead of using the database name as given.