provider/openstack: Per-network Floating IPs

This commit adds the ability to associate a Floating IP to a specific
network. Previously, there only existed a top-level floating IP
attribute which was automatically associated with either the first
defined network or the default network (when no network block was
used).

Now floating IPs can be associated with networks beyond the first
defined network as well as each network being able to have their own
floating IP.

Specifying the floating IP by using the top-level floating_ip
attribute and the per-network floating IP attribute is not possible.

Additionally, an `access_network` attribute has been added in order
to easily specify which network should be used for provisioning.