Define a separate dir for OVS DPDK vhostuser 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 defines the path so the dir could be created by the nova
formula [1] with needed permissions, and also used by linux.network [2]
and neutron formulas for proper configuration.

[1] https://gerrit.mcp.mirantis.net/#/c/11213
[2] https://gerrit.mcp.mirantis.net/#/c/10763
[3] https://gerrit.mcp.mirantis.net/#/c/10764/

Related-PROD: PROD-14413

Change-Id: Ibacffe8f31432dc24fd9fd81b159faadcddd062a
1 file changed