Support "databaseless" execution commands

# Current behavior

When a driver initiates a connection it gets passed the config and uses this straightforward. This creates a problem as soon as commands like `db:create` and `db:drop` are about to be executed. Those have no need to be actually be connected to a database directly and this in turn might result in issues while connecting to this database which may not exists already.
# Proposed solution

On commands that can omit the database parameter those are going to be stripped away by db-migrate before they actually reach the driver.
The drivers in turn have to assure that they handle the default behavior correctly. There are for example open items for postgres https://github.com/db-migrate/pg/issues/5.
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