further route table abstraction

added new private method to trim down repeatable code.  Return
CloudException for older deprecated methods for backwards compatability
(getRoutingTableForVlan and getRoutingTableForSubnet) but return null
(consistant with other resources) for new method (getRoutingTable -
uses routingtableid).  Ref#622.

Additionally returned originally functionality to
getRoutingTableForVlan that Closes #639