Introduce environment variable preset features

[jglover]

Changes introduce environment variable preset features (parity with tutum/mysql).

[jglover]

Adds support for the following environment variables & features from tutum/mysql:

MARIADB_USER - Sets the admin account username (defaults to 'admin')

MARIADB_PASS - Sets the admin account password (defaults to random string, see docker logs <CONTAINER_ID>)

STARTUP_SQL  - SQL to import at startup (needs to exist, can be mounted)

ON_CREATE_DB - Database to create at startup.

[jglover]

@tifayuki