Make it possible to change the master database name

[au-ee]

Checklist

• I have read the Contributing Guide
• I have checked to ensure this does not introduce an unintended breaking changes
• I have considered appropriate testing for my change

Description

I had to run a DbUp migration with a hosted PostgreSQL database at a cloud provider.
Their setup does not provide a database named postgres which DbUp requires.

In this change, I introduced a public static property named MasterDatabaseName which has the default value postgres. If the user does not set it explicitly, nothing changes. In my case, I could just set it to the name of an existing db name (which is defaultdb in my case).

References:

• I saw that there was a similar PR two yeary ago:

  EnsureDatabase.For: Fixed issue when postres database name is always 'postgres' DbUp#605

  This approach did not work out because people relied on havinig DbUp set the master db name. My PR should bring together the best of both worlds and therefore make DbUp usable with a wider variety of hosted PG variations.

• This change would also fix EnsureDatabase.For.PostgresqlDatabase doesn't work if database account cannot access postgres database #20

The base branch was changed.

[droyad]

Thanks @au-ee. A previous PR added the options argument and using that is more easily discoverable than a static.