Remove all existing cases of custom args for Task class ctors.

Tasks are instantiated in a managed way, in the execution engine.
So they must all have a set of known ctor args (currently context and workdir).

In some cases we allowed extra args, used only by tests. This change removes
those extra args and requires all Task classes to receive any extra information
via the context, even at test time.

This is for a couple of reasons:

1) Since the task cannot have extra args when run in the execution engine,
it's more realistic not to allow tests to do so.

2) We want to be able to change the Task base class's ctor args in the
future without having to modify dozens of subclasses. To do so, we'd
like to change all Task ctors to *args, **kwargs style. This change
is the first step towards that.

Note that we even disallow intermediate classes from having extra ctor args,
even if in theory that would be OK as long as all instantiable subclasses provide
those arguments. However it's simpler and more uniform, especially in the
*args, **kwargs style, not to allow this.
Conflicts:
	src/python/pants/backend/core/tasks/console_task.py
	src/python/pants/backend/core/tasks/filter.py
	src/python/pants/backend/core/tasks/listtargets.py
	src/python/pants/backend/core/tasks/what_changed.py
	src/python/pants/backend/jvm/tasks/dependencies.py
	src/python/pants/backend/jvm/tasks/ivy_resolve.py
	src/python/pants/backend/jvm/tasks/jar_publish.py
	src/python/pants/backend/jvm/tasks/jar_task.py
	src/python/pants/backend/jvm/tasks/jvm_compile/java/java_compile.py
	src/python/pants/backend/jvm/tasks/jvm_compile/jvm_compile.py
	src/python/pants/backend/jvm/tasks/jvm_compile/scala/scala_compile.py
	src/python/pants/backend/jvm/tasks/nailgun_task.py
	tests/python/pants_test/tasks/test_console_task.py
Dummy commit to provide message intended for previous commit.

Message for 1d68bf9d0c326883962ae8e0f05394c03682da54 should have been:

Change all Task subclass constructor args to (*args, **kwargs).

Unfortunately the message got messed up in a merge, and I only
discovered this after pushing, and didn't want to force-push a fix,
as that leads to trouble.
Migrate some reasonable default values from pants.ini into 'defaults' in the pants source