Add the database master upgrade target, and check for valid steps.
Worked out if master, now for extra testing.
Add a test for the upgrade targets.
Refactoring post review.
Merge pull request #321 from howbazaar/schema-upgrade-type

Schema upgrade type

Add an upgrade target for DatabaseMaster, that should be used for any db-schema changes.

When the machine agent runs the upgrade steps now, it also checks to see if it is the master, and if so, uses this to determine the steps.