Use python from venv for subunit-trace
Use python from venv for subunit-trace

If subunit installed from packages then subunit-trace goes to
/usr/bin/ dir with hasbang string #!/usr/bin/python

In this case even if called from venv it use system python
interpreter, and will not have access to modules from venv.

Change-Id: I9b152055c5ce958001e59908564a3549960a7df9
DavidPurcell/murano_temp,NeCTAR-RC/murano,NeCTAR-RC/murano,sajuptpm/murano,chenyujie/hybrid-murano,satish-avninetworks/murano,olivierlemasle/murano,NeCTAR-RC/murano,satish-avninetworks/murano,openstack/murano,satish-avninetworks/murano,DavidPurcell/murano_temp,satish-avninetworks/murano,openstack/murano,olivierlemasle/murano,DavidPurcell/murano_temp,olivierlemasle/murano,chenyujie/hybrid-murano,sajuptpm/murano,DavidPurcell/murano_temp,NeCTAR-RC/murano,olivierlemasle/murano