s1gw: enable the REST interface, fix wrong REST port
REST had been disabled because only nightly builds supported it. The latest stable release (v0.4.0) also supports the REST interface, so let's enable it unconditionally by removing the mp_rest_enable.
Also fix the REST port: mp_rest_port was incorrectly set to 8125 (the StatsD port) instead of the actual REST port 8080.
