provision: change the Provisioner interface

 * Added the method RemoveUnits, that takes the app and the number of
   units to remove
 * Changed the method RemoveUnit. Now it takes the unit to remove, not
   the application

Getting rid of a bug introduced by tsr 0.5.0, with unit-add and
unit-remove.
testing/provisioner: match the new Provisioner interface

- Add RemoveUnits
- Fix RemoveUnit
testing/provisioner: do not provision app with a unit

This was the old behavior of the Juju provisioner, now we live in a
world where provisioning a unit is very cheap, let's do that on demmand!
provision/docker: adapt to the new Provisioner interface

- added the method RemoveUnits
- fixed the behavior of the method RemoveUnit
app: fix tests broken by the recent change in the FakeProvisioner

See 61c8b4d3b5e7fa1989fe8c438a13a43f8061ca61 for more details on the
change.
api: fix tests broken by the recent change in the FakeProvisioner

See 61c8b4d3b5e7fa1989fe8c438a13a43f8061ca61 for more details on the
change.
testing/provisioner: fix AddUnits

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