Updated openstack/openstack
Project: openstack/taskflow  45e1e62774d75f9d92f27346631f06a0bc3047cf

Update engine class names to better reflect there usage

Rename the single threaded engine to be the serial engine which
better matches its entrypoint, do the same for the multithreaded
engine (renaming it to the parallel engine).

Change-Id: I6174b4f1936858c13eeee416bfa3836cf20a1350