Rename Task.__call__ to Task.execute

We'd like library interface to be nice and simple, which implies that
it does not involve too many underscores. Also 'execute' matches with
'revert' nicely.

Suggested-by: Anastasia Karpinska <akarpinska@griddynamics.com>
Change-Id: Iea72e6230a32665b4236a1e7d1dcc41bd56d0418