Instead of having the optional requirements example be a
example that is itself a unittest just move the example to be an
actual unit test that gets tested using the various engine types
and change the example to be something slightly different (but
shows the same kind of usage information).

Change-Id: Ia03a81a6be636c501a35e7e290f587f7d05f8b30

Optional arguments that are not explicitly required are being ignored
when arguments are being mapped based on inference from atoms' execute
method signatures.  This patch adds support for mapping optional
arguments in addition to required arguments.

Change-Id: I440c02dcd901a563df512e33754b13e3c05d4155

In examples and documentation it is better if we recommend
good practices for using the notifications and the good/better
practice would be to use the class defined constant instead of
using a raw string (in-case taskflow ever changes that constant
value sometime in the future).

Change-Id: Ied8fc88747f8635de4aa776095e7e0195d6043aa

Recreate a very simple eventlet utility module that
has only a few features; one function checks if eventlet is
available and if not raise an exception; and a constant that
can be used by calling code (such as tests or other optional
functionality) to check if eventlet is useable before
proceeding.

Change-Id: I32df0702eeae7c7c78972c9796156dd824b2f123

Prefix '_' to denote unused variables

Suffix '_' to avoid conflicting with Python built-in funcs

Change-Id: I4e0d0b8f88e5c93222fbd7f8dc7cf626923f738e

Prefer the non-deprecated 'oslo_utils' instead of
the namespaced 'oslo.utils' wherever it was previously
used.

Change-Id: I9a78150ef5266e1ff22147278162fe3cfe1b2e3f

Since the key 'old_state' is now standard across all notifications
sent from flows, retries or tasks we can safely use it in the listeners
without worrying about hitting key errors when it does not exist/is not
provided.

This also adds an example which shows how to use the dynamic logging
listener to view this information.

Change-Id: If4456440674347a3fe4972d9d0fa16ba8373ef9f