scapy: Fix conflict of scapy libraries

scapy: Fix conflict of scapy libraries

T-Rex requires a modified version of SCAPY library for python3
to function properly. It doesn't work with vanilla version of
scapy-python3 module, which is installed within vsperf environment by
pip for Xena. Currently vanilla scapy is imported by Xena and
enforced to T-Rex too, which causes following issues:

* missing implementation of Dot1AD causes failures
* broken multistream feature in T-Rex

VSPERF loads all Traffic Generator classes and thus all
imports performed at module level are performed. The solution
would be to import SCAPY module by Xena only in case, that Xena
traffic generator is really used. Please see JIRA for additional
information.

JIRA: VSPERF-566

Change-Id: I8018bc0126e752cc9f966252d17dadb6a5554b37
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>