hiding some of configurable settings

as requested in #2235 and #2244 this PR allow to add custom settings that will be hidden from the:
 - bin/omero web -h

    ```
OMERO.web configuration/deployment tools
Configuration:
    Configuration for OMERO.web takes place via the omero config commands. The
    configuration values which are checked are as below:
    Key                                  Default?  'Current value'
    omero.upgrades.url  ...
```
 - console output in Debug mode

    ```
DEBUG:omeroweb.settings:...
Validating models...
0 errors found
April 09, 2014 - 11:08:49
Django version 1.6, using settings 'omeroweb.settings'
Starting development server at http://0.0.0.0:8000/
Quit the server with CONTROL-C.
```

Currently ```omero.web.ui.* ``` are hidden. @will-moore @mtbc, @joshmoore  please let me know which of the existing settings should be hidden as well.