Extracted machine agent test upgrade functionality

This is in prepartion for future tests which require the same
functions.
cmd/jujud: restrict API logins during upgrades

Use the recently introduced API login validation functionality to only
allow client and local machine agent logins while upgrade steps are
running. The goal is minimise database activity during upgrades,
specifically for schema migrations.

As well as the unit tests, this change has been heavily tested
manually.
Merge pull request #142 from mjs/client-logins-during-upgrades

cmd/jujud: restrict API logins during upgrades

Use the recently introduced API login validation functionality to only allow client and local machine agent logins while upgrade steps are running. The goal is minimise database activity during upgrades, specifically for schema migrations.

As well as the unit tests, this change has been heavily tested manually.