[common] Add documentation and default for openstack_cmd

Author: elfiesmelfie

roles/common/README.md

@@ -58,7 +58,8 @@ For endpoint_tests.yml tasks:
       - polarion ID number for each test.
     common_endpoint_list
       - list of endpoints to validate
-
+    openstack_cmd
+      - The command used to run openstack
 
 For project_tests.yml tasks:
 

roles/common/defaults/main.yml

@@ -0,0 +1,2 @@
+---
+openstack_cmd: "openstack"