Get rid of hardcoded ESD key in global/shopware.con

After merging PR #8  I was thinking about creating a variable for the esd key to get rid of the hardcoded value in  `global/shopware.con`.

```
# sites-available/example.com.conf
set $shopware_esd_key '552211cce724117c3178e3d22bec532ec';`
```

```
# global/shopware.con
location ^~ /files/$shopware_esd_key/ {
    internal;
}
```

Unfortunately that seems not possible with plain Nginx config.
