Deployer facade; refactoring
Changes after review
Changes after review
Merge trunk
[r=dimitern] apiserver: Deployer facade; refactoring

This introduces the server-side Deployer API facade.
In addition to being composed by LifeGetter, Remover,
and PasswordChanger, it has WatchUnits method.

Also extracted the common ErrUnauthorized case into
its own common testing package and refactored other
facades to use it. Removed assertError and replaced
it with DeepEquals, because I think it stands out more
and reads better.

https://codereview.appspot.com/10949046/

R=fwereade, jameinel, themue