tox: Drop envdir

tox now always recreates an env although the env is shared using envdir
options.
~~~
$ tox -e genpolicy
genpolicy: recreate env because env type changed from
{'name': 'genconfig', 'type': 'VirtualEnvRunner'} to
{'name': 'genpolicy', 'type': 'VirtualEnvRunner'}
~~~

According to the maintainer of tox, this functionality is not intended
to be supported.
https://github.com/tox-dev/tox/issues/425#issuecomment-1011944293

Change-Id: I50ac742749b3a58cfd143ce92da72118e497f2af

tox.ini

@@ -69,7 +69,6 @@ setenv =
 
 [testenv:api-samples]
 install_command = {[testenv:functional]install_command}
-envdir = {toxworkdir}/shared
 setenv =
   GENERATE_SAMPLES=True
   PYTHONHASHSEED=0
@@ -127,7 +126,6 @@ commands =
 [testenv:genconfig]
 install_command = {[testenv:py3]install_command}
 sitepackages = False
-envdir = {toxworkdir}/pep8
 commands = oslo-config-generator --config-file=tools/config/cinder-config-generator.conf
 
 [testenv:genpolicy]
@@ -137,7 +135,6 @@ commands = oslopolicy-sample-generator --config-file=tools/config/cinder-policy-
 [testenv:genopts]
 install_command = {[testenv:py3]install_command}
 sitepackages = False
-envdir = {toxworkdir}/pep8
 commands = python tools/config/generate_cinder_opts.py
 
 [testenv:venv]
@@ -191,7 +188,6 @@ commands = sphinx-build -a -E -W -j auto -d releasenotes/build/doctrees -b html
 [testenv:gendriverlist]
 install_command = {[testenv:py3]install_command}
 sitepackages = False
-envdir = {toxworkdir}/venv
 commands = python {toxinidir}/tools/generate_driver_list.py
 
 [testenv:bandit]
@@ -203,7 +199,6 @@ commands = bandit -r cinder -n5 -x cinder/tests/* -ll
 [testenv:bandit-baseline]
 install_command = {[testenv:bandit]install_command}
 deps = bandit==1.6.0
-envdir = {toxworkdir}/bandit
 commands = bandit-baseline -r cinder -n5 -x cinder/tests/* -ii -ll
 
 [testenv:bindep]