reset-db to get all db parameters from the config

Refactor elements/boot-stack/bin/reset-db introducing a convenience
function to extract database name, user and password from the .db
configuration parameter and use those values to pass to os-db-create.

Includes an optional default dsn with a common db-password parameter,
and will forego the db create if neither the .db configuration nor
the default are provided.

Intent is to facilitate service level control on db passwords.

Change-Id: I6e109ce93e4c074dcb5fdfb0749799c869c25dd6
Merge "reset-db to get all db parameters from the config"