Allow for subworkflows to be added dynamically to a running workflow

In certain cases it is useful to be able to extend a workflow (via a subworkflow) due to a descion the running workflow has made.

For example:

Create volume.
   Setup volume partition table
   If using backing disk image:
      - Add subworkflow to clone image onto volume
   else:
      - Format with ext3
   Finalize volume
   Report back all ok

There are a few ways that this can be done, and care must be thought about, especially on how this will be resumed from and reverted from since dynamically added components of a workflow do not 'appear' in the workflow from the start so they can not be as easily tracked as a more static set of tasks.