cmd/juju: remove juju 1.10 compatibility hacks
Refactored NotifyWorker and added StringsWorker
Fixed 2 tests
Changes after review
Fixed dependent stuff
Fixed typos
Removed extra returns
Refactored MinUnitsWorker to use StringsWorker
Deployer uses StringsWorker
Changes after review
Merged parent branch
[r=dimitern] worker: Introducing StringsWorker

Implemented StringsWorker after the model
of NotifyWorker - waiting for changes from
api.StringsWatcher and calling the handler.

It was necessary to do some refactoring to
NotifyWorker and tests to avoid duplication.

Also improved tests.

https://codereview.appspot.com/12198044/

R=rogpeppe, themue
merge trunk;
[r=rogpeppe] cmd/jujud: remove compatibility hacks

After some discussion with fwereade, we've decided
that these are unnecessary (and broken too - EnsureAPIInfo
only works pretty much by coincidence, and won't
work when the unit agent is connecting to the API).

If anyone is still running 1.10, they can still
upgrade, but they'll need to do it by upgrading via 1.12.

https://codereview.appspot.com/12114043/
[r=dimitern] worker: Deployer/MinUnitsWorker use StringsWorker

Both MinUnitsWorker and Deployer now use the new
StringsWorker. Code refactored and simplified,
no other changes. Live tested on EC2.

https://codereview.appspot.com/12254043/

R=gz, rogpeppe, themue