Tweaked API layer of policy engine simulate

Previously, if the 'delta' parameter was passed it would be
considered True, even if 'delta' was assigned the value 'False'.

This change makes the 'delta' parameter True only if it has
a value assigned and it is something we can reasonably expect
to indicate true, e.g. "True", 1, or "1".

Change-Id: I35e60eefae0f4719b720b09a23a57e2be986e272