Fix creating portgroup provider net with no physical_network set

[OpenStack Infra]

https://review.openstack.org/310699
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
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commit 441dac8eedbc0b9748ad6691fda5af129e5a331c
Author: linb <email address hidden>
Date: Thu Apr 28 13:13:05 2016 +0800

    Fix creating portgroup provider net with no physical_network set

    Plugin can allow create portgroup provider network as bellow, but we
    should prevent admin creating a portgroup network without setting the
    physical_network attribute.

    CLI command:
    neutron net-create name --provider:network_type portgroup
    --provider:physical_network <port-group-moref>

    DocImpact

    Closes-Bug: #1576095
    Change-Id: I6f4977737df89ff1f49b7ae7c85aca90c5263cad