Implement the "juju environment jenv" command

[frankban]

This is used to import Juju generated jenv files into
the environments directory of the Juju home.

(Review request: http://reviews.vapour.ws/r/710/)

[frankban]

QA:

• juju bootstrap an environment;
• add a user to the environment:
  juju user add who --generate -o /tmp/myenv.jenv;
• import the resulting environment:
  juju environment jenv /tmp/myenv.jenv;
• the new default environment is called "myenv":
  juju switch;
• the "myenv" environment properly connects:
  juju status;
• ensure the imported environment contents make sense:
  cat ~/.juju/environments/myenv.jenv;
• try to import the environment another time: you should
  see an "environment already exists" error:
  juju environment jenv /tmp/myenv.jenv;
• do what suggested in the error message, i.e. pass a
  second parameter:
  juju environment jenv /tmp/myenv.jenv foo;
• the new default environment is called "foo" and
  it works: juju switch && juju status;
• check wrong paths are correctly handled e.g.:
  juju environment jenv /no/such,
  juju environment jenv /tmp;
• destroy the environment.

Done, thank you!

[frankban]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju