Added option for simple gateway

Simple gateway is used to provide external network connectivity without any
physical router, in test and development environments.

NOTES:

* pending spec tests
* Currently subnets, vrf, dest_net are added as params, which should be
parameters. I am working on to get a reliable way to get their values. Once that
is done, this can be changed to properties.
Merge pull request #23 from hkumarmk/vgw

Added option for simple gateway