Support "databaseless" execution commands

[wzrdtales]

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 db-migrate/pg#5.

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[charyorde]

+1. This should work since all we need is to establish a connection. From there on, migration tasks can leverage this connection to create migration-specific databases.