Fix #215 Neutron: Add subnetId to NetFloatingIP

[plombard]

PR description

Add subnetId field to NetFloatingIp. This field is used when creating a FloatingIp, to ensure it goes to a specific subnet.
https://docs.openstack.org/api-ref/network/v2/index.html?expanded=#create-floating-ip

Submitter checklist

Make sure that following is addressed to make the PR easier to process:

• If applicable, changes are covered by either a unit or a functional test.
• If applicable, changes ware verified manually against an OpenStack instance.
• If the change is API related, the PR description links to OpenStack API documentation of that particular endpoint/request (https://docs.openstack.org/api-quick-start/#current-api-versions).
• If the change concerns particular OpenStack service, its name is used as a PR name prefix (ex.: "Neutron: Add floatingIp port forwardings service").
• If the PR closes existing GitHub issue, PR name have prefix: Fix #NNN: .