Support for optional task arguments

We used to ignore optional task arguments (arguments of 'execute' method
with default value). With this change flow author can pass such
arguments to task by mentioning them in 'requires' and/or 'rebind'
task constructor parameters.

Change-Id: Ib92f87eb05623d30db6f1691d75c11376d45c87a
Merge "Support for optional task arguments"