Do not assume the default postgres db is called `postgres`

[tankyleo]

Digital ocean uses defaultdb, Amazon, Google, Microsoft use postgres, Heroku uses random name.

We require the default db name because that is how we check the existence of the database given in the configuration file, and create the given database as needed.

[tnull]

Couldn't we directly attempt to connect to the VSS database instead of the default database and conclude it's non-existent if we get an error kind Db back?

Nevermind, even if we know that the DB doesn't exist, we still need to successfully connect to create one.

Then we probably need to add a optional override parameter.

[tankyleo]

Fixed in #73