Change default SRIOV exclude_devices to ""

The current default for exclude_devices is null, which will
render suboptimally when using secret-based configmaps.
This changes it to "" (empty string) which will render correctly
when SRIOV is enabled.

Change-Id: Ib33181943d90278e1e1e9498bb0d77fd6c029ce5

neutron/values.yaml

@@ -1579,7 +1579,8 @@ conf:
         firewall_driver: neutron.agent.firewall.NoopFirewallDriver
       sriov_nic:
         physical_device_mappings: physnet2:enp3s0f1
-        exclude_devices: null
+        # NOTE: do not use null here, use an empty string
+        exclude_devices: ""
   dhcp_agent:
     DEFAULT:
       #(NOTE)portdirect: if unset this is populated dyanmicly from the value in