Add wait flag for delete cluster command

[Lazyshot]

Adding ability for a user to have the delete cluster command
wait until all resources are removed before exiting.

Issue #213

Note: Reopened due to issue with NodeGroup stacks must be fully deleted before the cluster stack deletion can be requested.

Checklist

• Code compiles correctly (i.e make build)
• Added tests that cover your change (if possible)
• All tests passing (i.e. make test)
• Added/modified documentation as required (such as the README)
• Added yourself to the humans.txt file

[richardcase]

This looks good to me.

It makes sense enforcing the wait for the node group to delete.

[errordeveloper]

It makes sense enforcing the wait for the node group to delete.

This is required because nodegroup stack imports values from cluster stack, which implies cluster stack cannot be deleted while there exists a stack that imports any of the exported values.