Make upgrade-gui a controller command.

[frankban]

Move the API client methods to the controller client.
Also implement a way to mock controller HTTP calls.

QA:

• bootstrap a controller;
• run juju help upgrade-gui and check that a -c (--controller) option is now available;
• run juju upgrade-gui --list and check that it properly returns a list of GUI archives available on simplestreams;
• run juju upgrade-gui to check that we already are at the most recent version (i.e. output should be "Juju GUI at version 2.1.10");
• run juju upgrade-gui 2.1.8 to switch to an older version;
• run juju gui and check that in this old version the login form was a bit weird (with really large text inputs);
• run juju upgrade-gui no-such: it should output "ERROR invalid GUI release version or local path "no-such"";
• run juju upgrade-gui 0.0.1: it should output "ERROR Juju GUI release version 0.0.1 not found";
• download jujugui-2.1.11.tar.bz2 from https://github.com/juju/juju-gui/releases;
• run juju upgrade-gui ~/path/to/jujugui-2.1.11.tar.bz2 and check that the upgrade suceeds;
• run juju gui --show-credentials: check that the login form is now rendered properly;
• log into the GUI and check that it works (for instance, deploy an application).

(Review request: http://reviews.vapour.ws/r/5619/)

[fwereade]

Nice changes, thanks. Code LGTM. Haven't QAed; EODing soon; can I hand that off to redir please?

[frankban]

Fabrice QAed that already, thank you all for the reviews!
$$merge$$

[jujubot]

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