Activate site-packages for tempest plugins

If site-packages is deactivated tempest plugins aren't visible.
Create a new tox entry point instead of activating it globally.

Change-Id: If6cd204d4c1e64cf7bcf79bd5d32d3c10a1b2559
Closes-bug: #1478559

tox.ini

@@ -40,6 +40,16 @@ commands =
   find . -type f -name "*.pyc" -delete
   bash tools/pretty_tox.sh '{posargs}'
 
+[testenv:all-plugin]
+sitepackages = True
+# 'all' includes slow tests
+setenv = {[tempestenv]setenv}
+         OS_TEST_TIMEOUT=1200
+deps = {[tempestenv]deps}
+commands =
+  find . -type f -name "*.pyc" -delete
+  bash tools/pretty_tox.sh '{posargs}'
+
 [testenv:full]
 sitepackages = {[tempestenv]sitepackages}
 setenv = {[tempestenv]setenv}