Big docs changes and few minor patches

Code changes do not break upstream functionality.
- kvm-vif-bridge is renamed to kvm-ifup-custom but a symlink is created until upstream Ganeti renames it too
- kvm-ifdown-custom is added but not used. Still dns updates and deletion of neighbor proxy entry is done by hooks.
- snf-network.ferm is added for defining firewall chains
- firewall is supported also for the bridged setup

Docs have changed a lot:
- Main page includes basic info
-  Two more pages have been added: one for routed setup and one for ebtables setup.