nginx config template improvement

As an addition to #1834 config should be more aware of settings in omero.web

This PR automatically adjust Django ```STATIC_URL``` if ```FORCE_SCRIPT_NAME``` is set and propagate it while generating nginx config:

```
bin/omero web config apache
bin/omero web config nginx
bin/omero web config nginx --system
```

In order to test it please set the following in various configurations (that make sense of course) including empty values and check if matching config was generated:

example:
```
bin/omero config set omero.web.static_url '/omero/static/'
bin/omero config set omero.web.force_script_name '/omero'
```
gives:
 - nginx

    ```
    server {
            ...
            location /omero/static {
            ...
            location /omero {
                ...
                fastcgi_split_path_info ^(/omero)(.*)$;
    ```
 - apache
    ```
    ...
    Alias /omero/static /YOUR_OMEROPATH/dist/lib/python/omeroweb/static
    Alias /omero "/YOUR_OMEROPATH/dist/var/omero.fcgi/"

    ```

Please note that static_url and force_script_name are independent
A couple things here. Firstly, `STATIC_URL` should always be set in accordance with the configuration in `settings.py`:

```
...
"omero.web.static_url": ["STATIC_URL", "/static/", str]
...
```

Is there a particular reason why you're checking for it to be missing or empty? I would think that if it is empty really bad things are going to happen.

You can also simplify and make more thorough the handling of trailing slashes by using `settings.STATIC_URL.rstrip('/')`.