add lxc.network.script.up configuration hook

add lxc.network.script.up configuration hook

This commit adds an configuration option to specify a script to be
executed after creating and configuring the network used by the
container. The following arguments are passed to the script:

	* container name
	* config section name (net)

Additional arguments depend on the config section employing a
script hook; the following are used by the network system:

	* execution context (up)
	* network type (empty/veth/macvlan/phys)

Depending on the network type, other arguments may be passed:

veth/macvlan/phys:
	* (host-sided) device name

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>