Bump VPP stable version: socksvr

Bump VPP stable version: socksvr

VPP changed the default socket path in:
https://gerrit.fd.io/r/c/vpp/+/21100

Instead of "default", explicit value is used now.
It coincides with VPP default one,
and is stored in Constants.py (except documentation).

The file name for robot side forwarded socket remains,
as the path leading to is is different anyway.

Change-Id: I0058d16794a3cd04411640c2c8d5fd4ee8cb997d
Signed-off-by: Vratko Polak <vrpolak@cisco.com>