Helpers to save flow factory in metadata

This change adds new helpers that, taken together, allow to resume
the flows having nothing but flow detail at hands. First one,
load_from_factory, gets flow factory function as a parameter and
saves its fully qualified name and arguments to flow metadata. Others
can be used to re-create the flow using that metadata, and load
it into engine.

Change-Id: Ia3cd989b3b0388ec0a9f09fe527f768eec5cc904
Merge "Helpers to save flow factory in metadata"