Fix tox coverange env
Fix tox coverange env

We're not setting the VIRTUAL_ENV environment variable, so 'tox -ecover'
is failing because patch_tox_venv uses the variable.

Change-Id: I3898d0dd1f8a9bf4216253bb2c7d8491e1e8ce56

JioCloud/oslo-incubator,JioCloud/oslo-incubator,JioCloud/oslo-incubator