Fix network info injection in pure IPv6 environment

get_non_legacy_network_template assumes that IPv4 is always available

So if i try to use Nova Network without any ipv4 using say
"nova network-create public --fixed-range-v6 fd8c:215d:178e:452:0:0:0:0/64"
then the deployment fails. So fix the code to ensure ipv4 is optional

Fixes LP# 1216178

Change-Id: Ib0c47a49debd4fed83d382a301687df23ca2ec0d
Merge "Fix network info injection in pure IPv6 environment"