Migrate to new NBI charm, and change in NGUI charm

- Fixes bug 1365
- Changes in the installer:
    - use site_url config to expose both NBI and NG-UI
    - use resources for images
- Changes in the NBI: some minor fixes
- Changes in the NG-UI: follow the same pattern as in the new set of
charms

Change-Id: I6a11009ddf9cd22689602b2a53ecf67f376830cb
Signed-off-by: David Garcia <david.garcia@canonical.com>