Change rVM's eth2 IPs From 10.252.0.x/24 to 0.0.0.0

Nova will handle the address assignment to iVMs (from fixed_range
10.252.0.x/24) and setup corrosponding routing table rules on rVMs
Change public_interface from eth3 to eth0

Currently only eth0 can forward Internet traffic. Also add some notes
on eth2, eth3, and public_interface.
Merge pull request #11 from changbl/working

Two changes (eth2 and public_interface)