Set the default config for the db backend from the available options

[0Tech]

The subject of the triage:

• chore: use default db backend among the available ones ostracon#212

[0Tech]

The patch would set the default config for the db backend from the available options.

Pros:

• config.toml will always have a working default value of db_backend.

Cons:

• The code will not tell you which value would be the default one. You must inspect build script.