Add clean.sh

clean.sh gets rid of all residue of running DevStack except installed
packages and pip modules.

And it eradicates rabbitmq-server and ts erlang dependencies as well as
the other RPC backends and databases.

Change-Id: I2b9a251a0a151c012bae85a5a2f9c2f72e7700be
diff --git a/lib/databases/postgresql b/lib/databases/postgresql
index 7d4a6c5..efc206f 100644
--- a/lib/databases/postgresql
+++ b/lib/databases/postgresql
@@ -10,6 +10,20 @@
 
 register_database postgresql
 
+# Get rid of everything enough to cleanly change database backends
+function cleanup_database_postgresql {
+    stop_service postgresql
+    if is_ubuntu; then
+        # Get ruthless with mysql
+        sudo aptitude purge -y  ~npostgresql
+        return
+    elif is_fedora; then
+        uninstall_package postgresql-server
+    else
+        return
+    fi
+}
+
 function recreate_database_postgresql {
     local db=$1
     local charset=$2