Add printing the plugin list after the plugin install

This commit just adds a sanity check output to lib/tempest. It will
use tempest list-plugins to print a table of installed plugins after
the pip install phase is run for any provided plugins. This will
enable users to check that the plugins they think they're running are
detected by tempest.

Change-Id: Icff286da6c68ec9a57f2288458976341bc095875

lib/tempest

@@ -600,6 +600,8 @@ function install_tempest {
     PROJECT_VENV["tempest"]=${TEMPEST_DIR}/.tox/tempest
     if [[ $TEMPEST_PLUGINS != 0 ]] ; then
         tox -evenv-tempest -- pip install $TEMPEST_PLUGINS
+        echo "Checking installed Tempest plugins:"
+        tox -evenv-tempest -- tempest list-plugins
     fi
     popd
 }