lxd/networks: Adds --dhcp-rapid-commit when dnsmasq version > 2.79
lxd/networks: Adds --dhcp-rapid-commit when dnsmasq version > 2.79

For DHCPv4 clients that support it this should speed up DHCP allocations by switching to a 2-way rather than 4-way protocol.

See RFC 4039.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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