install-guide: Added basic OS configuration

Still need to check a few things on Ubuntu & OpenSUSE

Change-Id: Ieb37505e0410bfc3735eeb201e8e7335aa32672a
initial work on neutron docs
Added instructions for configuring the external NIC
Fixes to Notes/Working OVS/GRE Notes

Tested the path [3-node]+neutron->ovs(+/-firewall)->gre, made
some fixes, and that doc now should work properly.