Issue #669 - Provide options to upgrade so that it will respect the stack_order.

--concurrency, -C - specifies that no more than NUM nodes can be converged at any given time.
--stack-order     - specifies that the group order specified in the plugin's stack_order should
 be respected. Each node in a group should be converged before moving on to the next group. If
 the concurrency option is also set it will further slice up the group so that no more than NUM
 nodes in a group will be converged at a given time.