Networking v2: Fix up subnetpools

[jtopjian]

During implementation of https://github.com/terraform-providers/terraform-provider-openstack/pull/243, we discovered that it is not possible to query by Prefixes.

The fields which Neutron allows for listing is dynamic, based on the fields of the resource itself. I guess a limitation of this is fields which are arrays.

Doing further testing, I found that it is also not possible to list by created_at or updated_at.

And in reviewing the code, I found a few other areas I missed during the original code review:

• The bool fields in ListOpts need to be *bool to allow values of false to be sent.
• The result struct's CreatedAt and UpdatedAt need converted to time.Time. This is probably the first occurrence I've seen where the timestamp doesn't need a special conversion.

[jrperritt]

closed by #789