Use absolute path for VENV
Use absolute path for VENV

Translation sync of horizon currently fails with:
/usr/local/jenkins/slave_scripts/common_translation_update.sh: line 320:
./tmp.iqbhMxlZtW/bin/pip: No such file or directory

Use an absolute path for $VENV so that it works just fine.

Change-Id: I9886f916c8d5377f2882e235bb9d70e2cbbfa0ce

Tesora/tesora-project-config,dongwenjuan/project-config,openstack-infra/project-config,openstack-infra/project-config,Tesora/tesora-project-config,dongwenjuan/project-config