Fix AttributeError exception in autoscaling

Use the nested() method instead of directly accessing the _nested
attribute which isn't always initialized.

Fixes bug #1220737
Change-Id: I9f36d98e91d5175b354e423d7159e690b1e5b696
Implement parallel delete

This is first patch in the series to implement parallel delete
and it includes following changes:

1. changing Stack.delete to use stack_task
2. In Resource.delete polling for check_delete_complete
3. Updating unit tests to use TaskRunner for resource.delete and
   resource.destroy

Blueprint parallel-delete

Change-Id: I8a96b1a956388372c29f9f45445f397467667110
parallelize volume deletion

This is second patch in the series to implement parallel delete
In this patch, parallelizing volume deletion and updating relevant
tests

Blueprint parallel-delete

Change-Id: Iaded5951618c2ac52c26d6d6437f152c658ad9cf
parallelize instance delete

This is third patch in the series to implement parallel delete.
In this patch, parallelizing instance deletion

Blueprint parallel-delete

Change-Id: I86d2e25901b330dcf4b0e512eec66fb71199727b
Merge "Implement parallel delete"
Merge "parallelize volume deletion"
Merge "parallelize instance delete"