remove duplicated code from uniter, use proxyupdater (renamed from machineenvironmentworker) across the board
address review
Merge pull request #1278 from fwereade/proxy-updater

rename MachineEnvironmentWorker, extract duplicated implementation from Uniter

...and as a driveby fix all the jujud tests that were cheerfully trying to write made-up proxy settings for the ubuntu user all the time.

(Review request: http://reviews.vapour.ws/r/593/)