(2.9) Refactor NIC configuration scripts

(2.9) Refactor NIC configuration scripts

Add new script vif-ganeti that could be used instead of
default vif-bridge in order NIC customization can be more
flexible.

Extract common code from kvm-ifup and insert it in a new
file net-common that is being sourced by kvm-ifup and
vif-ganeti and located under package lib dir (/usr/lib/ganeti/).

Install vif-ganeti properly with make under /usr/lib/ganeti too.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>