Add env flag to be able to change database engine

[halkeye]

The platform works great with postgres, just needed to make a tiny change to allow it.

• You can't set the charset to utf8mb4 with postgres, so only set it when its mysql (default)
• Allow database engine to be changed (MYSQL_ENGINE would be sticking to convention, but felt wrong).

I can add DATABASE_USER which defaults to MYSQL_USER which defaults to 'root' if you want make it more generic

[RyanNoelk]

Thanks for the PR will test this out when I have some spare time 👍

[coveralls]

Coverage increased (+0.03%) to 86.459% when pulling 70da7a3 on halkeye:allow_database_model into 80a691b on open-eats:master.