Bug 871475: add eth0 as default route but with metric 2. r=mwu

To enable telephony data connection emulation while keeping nomral
network function when data connection is not enabled, eth0 is reserved
as the default route but with its metric value set to a higher 2.
Merge pull request #9 from vicamo/bugzilla/871475/emulator-rmnet

Bug 871475: add eth0 as default route but with metric 2. r=mwu