tox: Reuse envdirs

This massively reduces the amount of time needed to install dependencies
and saves a lot of disk space to boot #winning

Note that it's not possible to use 'envdir = {[testenv:py27]envdir}' as
that syntax doesn't work with implicit attributes.

Change-Id: I229e1961a66db08be2d4f7235d2c3724e875f13b

tox.ini

@@ -54,6 +54,7 @@ commands =
 description =
   Run style checks.
 deps = {[testenv]deps}
+envdir = {toxworkdir}/venv
 commands =
   bash tools/flake8wrap.sh {posargs}
   # Check that all JSON files don't have \r\n in line.
@@ -64,6 +65,7 @@ commands =
 [testenv:fast8]
 description =
   Run style checks on the changes made since HEAD~. For a full run including docs, use 'pep8'
+envdir = {toxworkdir}/venv
 commands =
   bash tools/flake8wrap.sh -HEAD
 
@@ -71,6 +73,7 @@ commands =
 # TODO(melwitt): This can be removed when functional tests are gating with
 # python 3.x
 basepython = python2.7
+envdir = {toxworkdir}/py27
 setenv = {[testenv]setenv}
 commands =
   {[testenv]commands}
@@ -89,11 +92,13 @@ commands =
 # with python 3.5
 [testenv:functional-py35]
 basepython = python3.5
+envdir = {toxworkdir}/py35
 setenv = {[testenv]setenv}
 commands =
   {[testenv:functional]commands}
 
 [testenv:api-samples]
+envdir = {toxworkdir}/venv
 setenv =
   {[testenv]setenv}
   GENERATE_SAMPLES=True
@@ -104,14 +109,17 @@ commands =
   stestr slowest
 
 [testenv:genconfig]
+envdir = {toxworkdir}/venv
 commands =
   oslo-config-generator --config-file=etc/nova/nova-config-generator.conf
 
 [testenv:genpolicy]
+envdir = {toxworkdir}/venv
 commands =
   oslopolicy-sample-generator --config-file=etc/nova/nova-policy-generator.conf
 
 [testenv:genplacementpolicy]
+envdir = {toxworkdir}/venv
 commands =
   oslopolicy-sample-generator --config-file=etc/nova/placement-policy-generator.conf
 
@@ -120,6 +128,7 @@ commands =
 # section once we rely on coverage 4.3+
 #
 # https://bitbucket.org/ned/coveragepy/issues/519/
+envdir = {toxworkdir}/venv
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source nova --parallel-mode
@@ -133,6 +142,7 @@ commands =
   coverage report
 
 [testenv:debug]
+envdir = {toxworkdir}/venv
 commands =
   {[testenv]commands}
   oslo_debug_helper {posargs}
@@ -164,6 +174,7 @@ commands =
 [testenv:api-guide]
 description =
   Generate the API guide. Called from CI scripts to test and publish to developer.openstack.org.
+envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands =
   rm -rf api-guide/build
@@ -172,6 +183,7 @@ commands =
 [testenv:api-ref]
 description =
   Generate the API ref. Called from CI scripts to test and publish to developer.openstack.org.
+envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands =
   rm -rf api-ref/build
@@ -180,6 +192,7 @@ commands =
 [testenv:placement-api-ref]
 description =
   Generate the Placement API ref. Called from CI scripts to test and publish to developer.openstack.org.
+envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands =
   # Check that all placement api routes are in the documentation
@@ -190,6 +203,7 @@ commands =
 [testenv:releasenotes]
 description =
   Generate release notes.
+envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
 commands =
   rm -rf releasenotes/build
@@ -198,6 +212,7 @@ commands =
 [testenv:bandit]
 # NOTE(browne): This is required for the integration test job of the bandit
 # project. Please do not remove.
+envdir = {toxworkdir}/venv
 commands = bandit -r nova -x tests -n 5 -ll
 
 [flake8]