Support user defined port.

Support user defined port.

Allow specifying a port on which the 
GUI server starts listening for requests.

Tests: `make unittest`.

QA:
- bootstrap an environment;
- `make deploy`;
- wait for the unit to be deployed;
- visit the GUI URL, ensure port 80 redirects to 443 as usual
  and the GUI works as usual;
- switch to a customized port: `juju set juju-gui port=8080`;
- ensure the GUI is no longer reachable at port 80 or 443, 
  instead it works correctly if you visit `https://<GUI-URL>:8080/`;
- switch to a another customized port: `juju set juju-gui port=443`;
- this time the GUI is still listening on default port for HTTPS,
  but port 80 no longer redirects to HTTPS;
- switch to an invalid port: `juju set juju-gui port=80000`;
- the GUI refuses to do that, and it falls back to using
  the default configuration: ensure port 80 redirects to 443;
- switch again to a customized valid port: `juju set juju-gui port=12345`;
- ensure the GUI server is listening on that port;
- finally, unset the port value: `juju unset juju-gui port`;
- back to defaults: ensure port 80 redirects to 443;
- destroy the environment.

Done, thank you.

https://code.launchpad.net/~frankban/charms/trusty/juju-gui/custom-port/+merge/241802

(do not edit description out of merge proposal)

[frankban, 2014-11-14 13:54:15]

Please take a look.

[rharding, 2014-11-14 14:09:23]

LGTM no QA. Jeff and Francesco thanks! People are going to love getting this
feature into their hands.

[jeff.pihach, 2014-11-14 15:46:31]

LGTM QA OK
This looks great and works great, thanks for taking over!

[frankban, 2014-11-14 15:50:12]

*** Submitted:

Support user defined port.

Allow specifying a port on which the 
GUI server starts listening for requests.

Tests: `make unittest`.

QA:
- bootstrap an environment;
- `make deploy`;
- wait for the unit to be deployed;
- visit the GUI URL, ensure port 80 redirects to 443 as usual
  and the GUI works as usual;
- switch to a customized port: `juju set juju-gui port=8080`;
- ensure the GUI is no longer reachable at port 80 or 443, 
  instead it works correctly if you visit `https://<GUI-URL>:8080/`;
- switch to a another customized port: `juju set juju-gui port=443`;
- this time the GUI is still listening on default port for HTTPS,
  but port 80 no longer redirects to HTTPS;
- switch to an invalid port: `juju set juju-gui port=80000`;
- the GUI refuses to do that, and it falls back to using
  the default configuration: ensure port 80 redirects to 443;
- switch again to a customized valid port: `juju set juju-gui port=12345`;
- ensure the GUI server is listening on that port;
- finally, unset the port value: `juju unset juju-gui port`;
- back to defaults: ensure port 80 redirects to 443;
- destroy the environment.

Done, thank you.

R=rharding, jeff.pihach
CC=
https://codereview.appspot.com/174170043

[frankban, 2014-11-14 15:51:22]

Thanks for the reviews!