Remove network config hardcode for overcloud fixed range
Remove network config hardcode for overcloud fixed range

Current OVERCLOUD_FIXED_RANGE_CIDR and OVERCLOUD_FIXED_RANGE_GATEWAY
are defined but not used, also OVERCLOUD_NAMESERVER uses hardcode
'8.8.8.8' as default.

Change-Id: I0dfcac1438a7c3e686fda7589008a875f5ba4db7

rdo-management/tripleo,rdo-management/tripleo