lxc-net: add LXC_DHCP_PING boolean option

lxc-net: add LXC_DHCP_PING boolean option

Excerpt from dnsmasq(8):
By default, the DHCP server will attempt to ensure that an address in not
in use before allocating it to a host. It does this by sending an ICMP echo
request (aka "ping") to the address in question. If it gets a reply, then the
address must already be in use, and another is tried. This flag disables this check.

This is useful if one expects all the containers to get an IP address
from the LXC authoritative DHCP server and wants to speed up the process
of getting a lease.

Signed-off-by: Jonathan Calmels <jcalmels@nvidia.com>