Change mariadb plugin to one supporting single (share) db container instance

[webknjaz]

Motivation:

• having additional per-app db container makes the server too busy and eats far more RAM

Refs:

• https://github.com/krisrang/dokku-mariadb
  • newest fork: https://github.com/josegonzalez/dokku-mariadb
• https://github.com/ohardy/dokku-mariadb
  • newest fork: https://github.com/cphubs/dokku-mariadb (with shity commits)
• dokku/dokku-mariadb#68

[webknjaz]

@anxolerd mentioned plugins don't work (none of versions I've found), this may require forking and fixing them

[webknjaz]

Ref: dokku/dokku-redis#64

[webknjaz]

Ref: https://github.com/anxolerd/dokku-mariadb/tree/feat/68/single-instance-mode

[webknjaz]

@cleac this is patch work we were talking about