Omero in a box exec/user fixes

Some fixes for `omero-in-a-box`:

Previously the QUIT signal was being ignored by postgres since it was sent to the shell used to start postgres, similarly for nginx, e.g.:

```
2014-08-11 15:08:46,832 INFO waiting for pgsql, nginx, web, ssh to die
2014-08-11 15:08:50,318 INFO waiting for pgsql, nginx, web, ssh to die
2014-08-11 15:08:53,322 INFO waiting for pgsql, nginx, web, ssh to die
2014-08-11 15:08:56,325 INFO waiting for pgsql, nginx, web, ssh to die
2014-08-11 15:08:57,326 WARN killing 'web' (138) with SIGKILL
2014-08-11 15:08:57,327 INFO stopped: web (terminated by SIGKILL)
2014-08-11 15:09:00,319 INFO waiting for pgsql, nginx, ssh to die
2014-08-11 15:09:03,323 INFO waiting for pgsql, nginx, ssh to die
2014-08-11 15:09:06,326 INFO waiting for pgsql, nginx, ssh to die
2014-08-11 15:09:08,328 WARN killing 'nginx' (135) with SIGKILL
2014-08-11 15:09:08,329 INFO stopped: nginx (terminated by SIGKILL)
2014-08-11 15:09:08,330 INFO stopped: ssh (exit status 0)
2014-08-11 15:09:09,331 INFO waiting for pgsql to die
...
2014-08-11 15:11:06,560 INFO waiting for pgsql to die
2014-08-11 15:11:08,563 WARN killing 'pgsql' (133) with SIGKILL
2014-08-11 15:11:08,563 INFO stopped: pgsql (terminated by SIGKILL)
```
Using `exec nginx` fixes nginx, this doesn't work for postgres because `su -c` seems to start one or more nested shells. See a3f322e commit log for the full explanation. Similarly for omero and web.

Also fixed the downloaded server branch (`OMERO-5.0-latest-build` doesn't exist) and an incorrect postgres data dir in `wait_on_postgres`.