Show a link to the MAAS server.

[frankban]

When the provider is MAAS, display a link
to the MAAS management interface in the
header.

Also implemented the EnvironmentGet call
in sandbox mode.

[frankban]

QA:

• juju quickstart a local or ec2 env;
• switch to this branch:
  juju set juju-gui juju-gui-source="https://github.com/frankban/juju-gui.git maas-server-link";
• wait for the GUI to be ready;
• visit the GUI page, no MAAS link should be present in the header.

To see the link, visit the GUI on the juju-gui MAAS: I already
switched to this branch there, and you should see a link on the header.
Clinking the link the MAAS management interface should be open in
another tab.

[jujugui]

Test FAILed.
Refer to this link for build results: http://ci.jujugui.org:8080/job/juju-gui/2095/

[mitechie]

👍 QA ok on guimaas

[kadams54]

👍 Looks good!

[frankban]

Thanks for the reviews!

[jujugui]

Status: merge request accepted. Url: http://ci.jujugui.org:8080/job/juju-gui-merge