vpp: Define vppctl socket name

vpp: Define vppctl socket name

Newer versions of VPP require an explicit definition of CLI socket
file for proper functionality. Otherwise vppctl is not able to connect
to running vpp process and thus vsperf is not able to configure
and control vpp.

Change-Id: Id0df75fd0894e22b1b4d7fa05e16f49efe20d3ab
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>