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.
Is this right? AFAIK, the agent only gets restarted for certain types of errors (see connectionIsFatal in jujud/agent.go). Other errors just cause the worker to be restarted but not the whole agent.

I guess the most likely error here is a mongo connection error but I don't think it's as certain as the comment indicates. Maybe say "This error will most likely cause the agent to be terminated."