ovs-dpdk: add command-line log level param

Enable configuration of ovs-dpdk logging level via a command-line
parameter.

Logging level  may be set to a value between 1 (least loggging) to 8
(most verbose logging). There is a 1:1 correlation between these log
levels and Intel(R) DPDK log levels;

    * 1 = Print EMERGENCY messages
    * 2 = Print ALERT messages
    * 3 = Print CRITICAL messages
    * 4 = Print ERROR messages (default)
    * 5 = Print WARNING messages
    * 6 = Print NOTICE messages
    * 7 = Print INFORMATION messages
    * 8 = Print DEBUG messages

**  Note ** Higher log levels also report all lower-level messages.

Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>

Reviewed-By: Kevin Traynor <kevin.traynor@intel.com>
Reviewed-By: Stephen Finucane <stephen.finucane@intel.com>
Change-Id: I8b830b2ef104456d8cf648ff8a18c62df419518e