support public network

util/network_definiton.rb:
templates/libvirt_network.erb:
 - support bridge type definition

util/vm_definition.rb:
 - accept bridge type interface

action/network.rb:
driver/driver.rb:
 - activate public network
 - handle 'vagrant{N}' virtual bridge

Signed-off-by: Hiroshi Miura <miurahr@linux.com>