Add tempest-list-plugins

Add the tempest-list-plugins cmd.

bp list-plugins

Change-Id: If28311bc2e8d29a97ee46d7d73edba2a93aed7ce
diff --git a/requirements.txt b/requirements.txt
index 469b294..e5990ba 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -24,3 +24,4 @@
 tempest-lib>=0.12.0
 PyYAML>=3.1.0
 stevedore>=1.5.0 # Apache-2.0
+PrettyTable<0.8,>=0.7