Fix the postbuildscript documentation

The docstring for postbuildscript publishers stated that onsuccess
would default to False and onfailure defaulting to True. However,
if you look below around line 2523 you'll see it's the opposite.

Update the docstring to be correct and keep the behaviour of only
running onsuccess by default.

Change-Id: Ib98095fe0d741ef3afb6d4a8893568dae9be103d
Merge "Fix the postbuildscript documentation"