lxd/device/device/utils/network: Sets ARP probe timeout based on context deadline in isIPAvailable
lxd/device/device/utils/network: Sets ARP probe timeout based on context deadline in isIPAvailable

Also ensures there's always a context deadline even if one not provided by setting up a 500ms default one.

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

hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd