lbaas v2 NSX-v driver needs to improve handling Bad edge-bass request which causing ServerFault

[alex kang]

NSX-v does not support multiple tenant on same subnet.

So when creating load-balancer for other tenant by admin, there will be no edge-lbaas available for this tenant.

In our current implementation, this causing Bad edge-lbass response from backend and NSX-v driver throws Server Fault.

It is better that it can generate user-level error and not create the load-balancer at all.

## tempest test trace example:
=======================
ft7.4: vmware_nsx_tempest.tests.nsxv.api.lbaas.test_load_balancers_admin.LoadBalancersTest.test_create_load_balancer_missing_tenant_id_for_other_tenant[id-37620941-47c1-40b2-84d8-db17ff823ebc,smoke]_StringException: Empty attachments:
  stderr
  stdout

pythonlogging:'': {{{
2016-08-18 12:18:20,319 18865 INFO [tempest.lib.common.rest_client] Request (LoadBalancersTest:test_create_load_balancer_missing_tenant_id_for_other_tenant): 500 POST http://10.34.57.68:9696/v2.0/lbaas/loadbalancers 0.815s
2016-08-18 12:18:20,320 18865 DEBUG [tempest.lib.common.rest_client] Request - Headers: {'Content-Type': 'application/json', 'Accept': 'application/json', 'X-Auth-Token': '<omitted>'}
        Body: {"loadbalancer": {"vip_subnet_id": "18ba6dc8-d313-473f-a48c-8f8f66894bee"}}
    Response - Headers: {'status': '500', 'content-length': '179', 'content-location': 'http://10.34.57.68:9696/v2.0/lbaas/loadbalancers', 'date': 'Thu, 18 Aug 2016 19:18:20 GMT', 'content-type': 'application/json; charset=UTF-8', 'connection': 'close', 'x-openstack-request-id': 'req-f648433e-d554-4c2f-a869-6153e56a13a7'}
        Body: {"NeutronError": {"message": "Driver error: Bad edge-lbaas request: No suitable Edge found for subnet 18ba6dc8-d313-473f-a48c-8f8f66894bee.", "type": "DriverError", "detail": ""}}
2016-08-18 12:18:20,321 18865 INFO [vmware_nsx_tempest.tests.nsxv.api.lbaas.test_load_balancers_admin:LoadBalancersTest] Finished: test_create_load_balancer_missing_tenant_id_for_other_tenant
}}}

Traceback (most recent call last):
  File "/opt/devtest/vmware-nsx/vmware_nsx_tempest/tests/nsxv/api/lbaas/test_load_balancers_admin.py", line 83, in test_create_load_balancer_missing_tenant_id_for_other_tenant
    vip_subnet_id=self.subnet['id'])
  File "/opt/devtest/vmware-nsx/vmware_nsx_tempest/tests/nsxv/api/lbaas/base.py", line 145, in _create_load_balancer
    lb = cls.load_balancers_client.create_load_balancer(**lb_kwargs)
  File "/opt/devtest/vmware-nsx/vmware_nsx_tempest/services/lbaas/load_balancers_client.py", line 37, in create_load_balancer
    return self.create_resource(uri, post_data)
  File "/opt/devtest/mitaka-tempest/tempest/lib/services/network/base.py", line 60, in create_resource
    resp, body = self.post(req_uri, req_post_data)
  File "/opt/devtest/mitaka-tempest/tempest/lib/common/rest_client.py", line 270, in post
    return self.request('POST', url, extra_headers, headers, body, chunked)
  File "/opt/devtest/mitaka-tempest/tempest/lib/common/rest_client.py", line 664, in request
    resp, resp_body)
  File "/opt/devtest/mitaka-tempest/tempest/lib/common/rest_client.py", line 828, in _error_checker
    message=message)
tempest.lib.exceptions.ServerFault: Got server fault
Details: Driver error: Bad edge-lbaas request: No suitable Edge found for subnet 18ba6dc8-d313-473f-a48c-8f8f66894bee.