Added upgradeWorkerContext.IsUpgradeRunning()

This cleans up some previous uses of the UpgradeCompleted channel and
will be helpful for upcoming changes to the upgrader worker.
worker/upgrader: allow downgrades for aborted upgrades

In some cases upgrades will need to be aborted and the best course of
action will be to restart in to the previous agent version. This
revision modifies the upgrader worker so that it will allow downgrades
to the pre-upgrade tools version, but only if an upgrade is still in
progress. This allows the upgrade-steps worker to trigger a downgrade
to the previous agent version by changing the environment's
agent-version.
cmd/jujud: downgrade agent-version if a state fails to come up for upgrade

If during an upgrade one or more state servers fail to signal it is
ready for upgrade, the master state server will modify the environment
agent version to trigger a downgrade to the previous agent version.

So that the upgrader worker will accept downgrades to the previous
agent version (it normally won't), upgradeWorkerContext's
IsUpgradeRunning method is now passed to the upgrader
worker. Downgrades to the previous agent version are allowed if an
upgrade is in progress.
cmd/jujud: added wrench for state server upgrade wait

To support manual testing of a state server failing to say it's ready
for upgrade.
Merge pull request #555 from mjs/upgrader-allow-limited-downgrade

Support downgrades to previous version in the case of aborted upgrades

In some cases upgrades will need to be aborted and the best course of action will be to restart to the previous agent version. This PR modifies the upgrader worker and machine agent so that it will allow downgrades
to the pre-upgrade tools version, but only if an upgrade is still in progress. This allows the upgrade-steps worker to trigger a downgrade to the previous agent version by changing the environment's agent-version.