Rework CreateConnection of Moryx.Model

[dbeuchler]

As described and uncovered here: Establishing a database connection without a database-name makes no sense:
#584 (comment)

• includeModel on CreateConnection: maybe rename to useAdminDatabase or rename method to CreateTestConnection
• maybe create separate method for CreateTestDatabaseConnectionString which can be overridden in Npgsql/ms-sql

This ticket requires 5 minutes more brainpower but should be easy to solve.