Update root_password example to use ''

The value of False does not work for me on Ubuntu Vivid. Using '' everything works as expected.

pillar.example

@@ -2,7 +2,7 @@ mysql:
   server:
     # Use this account for database admin (defaults to root)
     root_user: 'admin'
-    # root_password: False - to have root@localhost without password
+    # root_password: '' - to have root@localhost without password
     root_password: 'somepass'
     user: mysql
     # If you only manage the dbs and users and the server is on