This change allows the following use cases:

1. Using an already-configured MariaDB / MySQL server / Cluster
2. Using already-created DB users, without requiring root DB access.

Update: added external mariadb precheck

Change-Id: I78b0d178306d7c5293b0bf53e445f19f18b4b824
Implements: blueprint external-mariadb-support.
Closes-Bug: #1603121