Remove runtime version checking of openvswitch

Currently, ovs_lib specifically checks openvswitch kernel/module/userspace versions at runtime when VXLAN is enabled. In a world where distributions frequently backport features into previous versions, this practice fails. Dependencies are a thing best handled by packaging. Instead, the runtime check should be removed from neutron and a separate script created that specifically tries to create/remove a vxlan port with openvswitch. This will give deployment tools, including devstack, a way to check for feature availability without polluting the neutron codebase.