Make init container SQL config more generic

This requires changes to the database secrets, which now have to have
an item with a key that matches the value of a line in each
`sql*.conf.ext` file formatted as:

> # dbname: <KEY>

Each item must additionally contain a "host" value as well as the
previously required "username" and "password" values.  Optionally
a "dbname" value could be supplied to override the database name in the
items's key.