CI: Fix IP address detection in baremetal compute test

The 'openstack server show <server> -f value -c addresses' command
previously had output like this:

    <network name>=<IP>

Now it shows a Python output like this:

    {'<network name>': ['IP']}

This broke the parsing of the command output when determining which IP
address to use to access a bare metal instance via SSH.

This change fixes the issue by querying the server's port in Neutron,
and using the fixed IP address.

Change-Id: I55b5f185fb7136d3c6fa565aa46598f21c94eb43