Pxe nodes are not provisioned, mgmt network is not required for them.
Provide new domain option 'mgmt_attach' which one can use to remove mgmt
interface from VM. Default behaviour is not changed.
Also take into account bridge interfaces when set boot order, to support
scenario when we would like to use vagrant VM to pxe boot physical nodes
in existing network.
This is required because in current versions of libvirt, it is not
possible to specify a boot order when attaching a device; therefore we
can only parse the entire domain XML after all devices have been created
and then assign boot ordering according to the Vagrantfile
specification. This allows us to specify exact boot order for hd, cdrom
and network.
