This commit makes some quick updates to the port attributes to make them
more intuitive:
* `security_groups` to `security_group_ids`: since the port is expecting
IDs and not security group names like in other areas of OpenStack.
* `admin_state_up`: change to Boolean to match this same attribute on
other resources.
* `fixed_ips` to `fixed_ip`: while multiple `fixed_ip` blocks can be
specified, only one fixed IP can be specified in each block.
Joe Topjian