Set allow_overlapping_ips to true

This needs to be disabled only when using nova security groups, but we
are using neutron security groups.
Make allow_overlapping_ips setting depend on use_namespaces

Note that we always set use_namespaces to true anyway, but at least this
makes the dependency between the two options more visible.
Merge pull request #37 from vuntz/allow_overlapping_ips

Set allow_overlapping_ips to true