Don't load non json body
Don't load non json body

Some neutron POST APIs don't return response body, for instance,
dhcp-agent-network-add and l3-agent-router-add. However,
BaseNetworkClient passes all response body to json.loads.
This patch add a check for response body.

Change-Id: I7a4c565f9e067e1771bf6ec85c9c0486937a2cef
Related-Bug: #1555921

masayukig/tempest,sebrandon1/tempest,vedujoshi/tempest,masayukig/tempest,sebrandon1/tempest,Tesora/tesora-tempest,Juniper/tempest,Juniper/tempest,vedujoshi/tempest,cisco-openstack/tempest,openstack/tempest,Tesora/tesora-tempest,cisco-openstack/tempest,openstack/tempest