Raise error on nova-api if missing subnets/fixed_ips on networks/port

Previously, if one would launch an instance on a network that did not contain
any subnets or on a port that did not have a fixedip the instance would fail
to boot resulting in a SecurityGroupCannotBeApplied  on the nova-compute node.
This patch adds checks to validate networks so it fails on nova-api and
the user will be able to see why from the response.

Note: a patch to fix the neutron port-security-enabled extension will
follow which allows one to boot instances on networks without subnets
and ports without fixedips.

Change-Id: I104d5234b34726e5f5819fc2a81eef55e3c70f42
Closes-bug: 1252410
Merge "Raise error on nova-api if missing subnets/fixed_ips on networks/port"