Update suites to use github.com/juju/testing.FakeHomeSuite.

Update dependencies to the last github.com/juju/testing revision.
Create JujuOsEnvSuite to host the existing logic shared in 
BaseSuite and FakeJujuHomeSuite.

Remove the (Make)FakeHome code from juju: it is now placed in
github.com/juju/testing.

The remaining bits are mechanical changes.