Add neutron_plugin_options section in tempest.conf

Add neutron_plugin_options section in tempest.conf

It avoids skipping the next 2 tests [1]:
neutron_tempest_plugin.api.admin.test_agent_availability_zone.AgentAvailabilityZoneTestCase.test_agents_availability_zone
neutron_tempest_plugin.api.admin.test_networks.NetworksTestAdmin.test_create_tenant_network_vxlan

[1] http://artifacts.opnfv.org/functest/functest-opnfv-functest-smoke-latest-neutron-tempest-plugin-api-run-498/results/neutron-tempest-plugin-api/tempest-report.html

Change-Id: I83f3f44302336045e47514e0aa0b67737161f0e1
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>