Use a separate dir for vhost_user sockets

Currently when OVS-DPDK is enabled, instances fail to spawn
due to permissions mismatch, see nova bug for details:
https://bugs.launchpad.net/nova/+bug/1670950

This patch updates openvswitch agent config to use a separate
directory for the sockets. Nova formula patch:
https://gerrit.mcp.mirantis.net/#/c/11213/ takes care
of creating the dir with proper permissions.

Corresponding reclass-system patch is:
https://gerrit.mcp.mirantis.net/13307/

Related-PROD: PROD-14413

Change-Id: I9fdc12ec4c99a517a1ea4e7aba86692529b76c96
diff --git a/README.rst b/README.rst
index c8ff890..0a06ec4 100644
--- a/README.rst
+++ b/README.rst
@@ -660,6 +660,7 @@
       compute:
         version: mitaka
         dpdk: True
+        vhost_socket_dir: /var/run/openvswitch
         backend:
           engine: ml2
           ...