Skip None values in fixed_ips when creating Port

Neutron create port doesn't like when a None value is passed for
fixed_ips data, the value needing to be skipped instead. This patch
handles that problem for ip_address and subnet_id, as testing showed
that subnet_id wasn't mandatory.

Change-Id: I18a06cbad456eb75c41c299bc4c51c704d9dabc3
Closes-Bug: #1224472
Merge "Skip None values in fixed_ips when creating Port"