Added script for doing tempest plugin sanity

It will help to catch tempest plugin config and dependency issues
with tempest.
sh ./tools/tools/tempest-plugin-sanity.sh or
tox -e plugin-sanity-check will invoke the same.

Change-Id: If42561bfc1d6dcffbb6537634d7769ef90fab21d
diff --git a/tox.ini b/tox.ini
index 892f834..2120818 100644
--- a/tox.ini
+++ b/tox.ini
@@ -185,3 +185,9 @@
 # separately, outside of the requirements files.
 deps = bindep
 commands = bindep test
+
+[testenv:plugin-sanity-check]
+# perform tempest plugin sanity
+whitelist_externals = bash
+commands =
+  bash tools/tempest-plugin-sanity.sh