do not provide a default for network_public_network

If quickstack::nova_network::compute is called with network_public_iface
set but no value provided for network_public_network, the default value
for network_public_network will be used in the call to find_nic(), which
will in almost all cases cause find_nic() to return an empty value.

This patch resolves that problem by removing the default setting for
network_public_network.

Resolves: rhbz#1127736
Merge pull request #343 from larsks/bz/1127736

do not provide a default for network_public_network