add ability to lookup multiple cloudformation stacks by tag

[Guslington]

At the moment a scheduled job has to be created for each stack that needs to be stopped and started or other additional automation has to be build to facilitate the stack lookup.

This functionality could be added to cfn_manage to lookup up a group of stacks based upon a common tag key and value placed on the stack

for example

cfn_manage stop-environment --tag StopStack=true
cfn_manage start-environment --tag StartStack=true