Change ovs_base neutron plugin to use vercmp

This plugin was using a deprecated function, vercmp_numbers(),
that wasn't actually working properly because the call to
'deprecated' at the beginning was causing garbage to be
returned to the caller.  For example, this was always in
stack.sh.log when using OVS:

.../lib/neutron_plugins/ovs_base: line 57: [: too many arguments

Update to use vercmp() like all other users in devstack, and
remove all the old code.

Change-Id: I352362cf59e492fa9f7725190f0243f2436ac347
diff --git a/lib/neutron_plugins/ovs_base b/lib/neutron_plugins/ovs_base
index 59c7737..95f4663 100644
--- a/lib/neutron_plugins/ovs_base
+++ b/lib/neutron_plugins/ovs_base
@@ -54,7 +54,7 @@
     local kernel_major_minor
     kernel_major_minor=`echo $kernel_version | cut -d. -f1-2`
     # From kernel 3.13 on, openvswitch-datapath-dkms is not needed
-    if [ `vercmp_numbers "$kernel_major_minor" "3.13"` -lt "0" ]; then
+    if vercmp '$kernel_major_minor' '<' '3.13'; then
         install_package "dkms openvswitch-datapath-dkms linux-headers-$kernel_version"
     fi
 }