workerpool: Change signature of AddTask function to not use *args

By changing it to a normal parameter, which must be a sequence, we can
start using keyword parameters.

Before this patch all arguments to “AddTask(self, *args)” were passed as
arguments to the worker's “RunTask” method. Priorities, which should be
optional and will be implemented in a future patch, must be passed as a keyword
parameter. This means “*args” can no longer be used as one can't combine *args
and keyword parameters in a clean way:

>>> def f(name=None, *args):
...   print "%r, %r" % (args, name)
...
>>> f("p1", "p2", "p3", name="thename")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 TypeError: f() got multiple values for keyword argument 'name'

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>