Default to use MySQL persistence instead of File persistence for Fedora

[joecorall]

Provide the necessary environment variables to create a database for fedora.

• FCREPO_PERSISTENCE_TYPE is defined in the fcrepo docker image. This configured fedora to use mysql as the persistant storage and is the recommended setting for production systems: https://wiki.lyrasis.org/display/FEDORA6x/Database+Cache
• DB_HOST and DB_PORT are inherited from the base image, and have no default value in the fcrepo container. Those values are needed by the script that creates the database.

Related to #21

[rosiel]

This appears to be working much better. WRT the problems in #21,

• There's a db in mariadb called fcrepo and its' getting populated.
• After adding a node, i can see it in Fedora
• I can add media to file fields that write to Fedora.

I assume this was just an oversight before? I'm hesitant to merge because I don't understand why this stopped working or whether it wasn't working to begin with. @nigelgbanks any insight?

[nigelgbanks]

Could be a volume / restart thing?

I don't know what Fcrepo is doing internally for its file backend. For example, it may create a file as a lock mutex, to control concurrent access to its file backed database. If restarted, the file lock may still be present, on the volume as Fcrepo rarely shutdowns gracefully, but the process using it as a lock has disappeared, so no new connections to its file backed database could happen? Or a needed file isn't captured by a volume and, as such, a restart causes some sort of data loss / race condition.

Though that's just speculation....

This is definitely safe to merge though.