Use ops.jujucontext._JujuContext

[tonyandrewmeyer]

We should be able to simplify some of the code by using the new ops.jujucontext._JujuContext class to manage mocking the environment variables that normally come from Juju.

There's some compatibility code added in #178 that can be cleaned out and replaced by requiring the newer version of ops and making proper use of a _JujuContext object.

Maybe we want to do this and #125 together, or do that one first and then this one?

Relevant ops PR: canonical/operator#1313