Add restriction for NSX settings and 'nsx' value to net_l32_provider

There is a neutron parameter NeutronConfig.net_l32_provider that was not used to reflect that NSX plugin is in use .
It's used now to reflect what Neutron plugin is in use.
Restriction for NSX settings is added to fixture.

Closes-Bug: #1346928

Change-Id: I7152bae4f216b8efe47a24d61529363e8955ee70
Merge "Add restriction for NSX settings and 'nsx' value to net_l32_provider"