Also, rename instance_id field to proper port_id.
Before this change, when instance_id was passed from the Instances
page in the URL, the information would be lost on form submission,
and if the form contained an error, the it would be redisplayed with
the port drop-down containing all ports from all instances. This also
made submitting this form slow, as the drop-down would be populated
with all ports on form validation.
It also creates a bug, where if there are more instances than Nova's
pagination allows, the ports from newer instances would no longer
appear in the drop-down, and the form would fail to validate, even
though the choice of port on initial form was correct.
Closes-bug: #1920010
Change-Id: I3ab26c19dc9ea1ed23fcff790d0db919039099eb
3d82d57353