Fix and improve cross-project functional testing

Make sure stestr is executed using the python interpreter set inside
the virtualenv, otherwise the libraries installed there may not
found. This is due to the fact that (stestr default shebang points
to system python when installed through devstack and the functional
tox target uses that version thanks to sitepackages=True.
The sitepackages option  must be used to find a few system ceph/python
libraries not available on pypi.

Setting PATH is not really needed nowadays, so remove the custom
workaround. It is worth noting that the hack would not be needed anymore
and PATH could be set through setenv (extending {env:PATH}).

As cinderlib changes can break cinder and cinder-tempest-plugin,
and they indeed did it in the past, so ensure that everything works
by running cinder-tempest-plugin-lvm-lio-barbican.

Change-Id: I40ef42b4224521abed00b3b210418ec96c1116ea

.zuul.yaml

@@ -11,6 +11,7 @@
 
         - cinderlib-lvm-functional
         - cinderlib-ceph-functional
+        - cinder-tempest-plugin-lvm-lio-barbican
     gate:
       queue: integrated
       jobs:
@@ -20,6 +21,7 @@
 
         - cinderlib-lvm-functional
         - cinderlib-ceph-functional
+        - cinder-tempest-plugin-lvm-lio-barbican
     post:
       jobs:
         - publish-openstack-python-branch-tarball

tox.ini

@@ -39,7 +39,6 @@ passenv = CL_FTEST_POOL_NAME
 setenv = OS_TEST_PATH=./cinderlib/tests/functional
          CL_FTEST_CFG={env:CL_FTEST_CFG:{toxinidir}/cinderlib/tests/functional/lvm.yaml}
          CL_FTEST_ROOT_HELPER={env:CL_FTEST_ROOT_HELPER:{toxinidir}/tools/virtualenv-sudo.sh}
-         ADD_PATHS=/usr/local/sbin:/usr/sbin
 
 sitepackages = True
 # Not reusing py37's env due to https://github.com/tox-dev/tox/issues/477
@@ -48,12 +47,13 @@ sitepackages = True
 # Must run serially or test_stats_with_creation may fail occasionally
 commands =
     find . -ignore_readdir_race -type f -name "*.pyc" -delete
-    # Tox has a bug and it ignores the PATH set in setenv, so we work around it
-    bash -i -c "PATH=$PATH:$ADD_PATHS stestr run --serial {posargs}"
-    stestr slowest
+    # run stestr through python -m so that the python from the virtualenv
+    # is used; stestr's shebang may point to system python, thus breaking
+    # the lookup of modules from the venv.
+    python -m stestr run --serial {posargs}
+    python -m stestr slowest
 
 whitelist_externals =
-  bash
   find
   stestr