Fix network_vlan_ranges parameter for OVS plugin

Class neutron::plugins::ovs expects string value in network_vlan_ranges parameter,
but validate_network_vlan_ranges will fail when value is comma separated list
of valid values. This patch is fixing this problem

Change-Id: If5b8ef59a600b3204415853ad796fb0537fefe4a
(cherry picked from commit 3d8d9fafec004b0debb2af8a70f4cd77d67ce04d)
Merge "Fix network_vlan_ranges parameter for OVS plugin" into stable/havana