Using the setDoc for the command documentation.
Added tests for the help output.
Merge pull request #559 from TheMue/juju-set-use-written-doc

Simple change in juju set to use the already written documentation

A variable named setDoc with the documentation already existed a longer time. But it never has been used as command documentation.

Fixes https://bugs.launchpad.net/juju-core/+bug/1359187