Fix Horizon integration job: permissions

Trying to run Tox as is results in the following error on Jenkins runs:

py.error.EACCES: [Permission denied]: mkdir('/opt/stack/new/horizon/.tox',)

The tox command should explicitly be run with the stack user.

Change-Id: I1b322b7e5dde7fd89f14447e65b2cf5dfd4ef900
Merge "Fix Horizon integration job: permissions"