ENI fixes for v4/v6 ips. DHCP test fixes.

Updates the ENI format so it creates multiple config sections
for a single os-net-config interface object if both v4 and v6
IP addresses are defined.

Also, fixes several of the test so that DHCP is not enabled on
OvsPorts. If an OVS port is part of a bridge that is itself using
DHCP we don't also need to run DHCP on the OvsPort interface.