Finish removing teuthology-worker

Finish removing teuthology-worker

The dispatcher and supervisor were added in #1546, but code was copied and
pasted into the new modules, leaving the worker untouched. Also untouched were
the unit tests, meaning that the dispatcher and supervisor were never unit
tested. As the copied code changed, the dispatcher and supervisor were not being
tested for regressions, while the worker - which wasn't being anymore - had
passing unit tests, giving some false sense of security.

This commit removes the old worker code, and adapts the old worker tests to
apply to the dispatcher and supervisor. It also splits out teuthology-supervisor
into its own command.

Signed-off-by: Zack Cerza <zack@redhat.com>