Fix building quantumapi network model with network list

Bug 1171636 occurs when _build_network_info_model() receives a list
of networks that does not fully map all of an instance's ports.
It crashes in _ensure_requested_network_ordering() when the list of
all ports is sorted by the subset of networks provided.

This change simplifies the early part of that function to only
consider ports within the networks provided, and to always sort those
ports in network order. This allows us to remove some error path
code from the loop below that iterates over the ports (which contained
a fatal typo, by the way) which is now dead code.

The affected code is really untested at the moment, due to the
structure of the method, making it hard to add a unit test to cover
this fix. In order to make the change clear from the existing code,
this fix is provided here first, and the following patch restructures
the method and add tests for this change and the rest of the surrounding
code.

Fixes bug 1171636

Change-Id: Idf519a88415844451b18959b38cc48ce92c090d1
(cherry picked from commit 08c7cf5c67a75dea9b453bd915a0cd55e67c4adc)
Merge "Fix building quantumapi network model with network list" into stable/grizzly