provisioner: added method to return app units.

related to #806.
docker: implemented provisioner.Units.

related to #806.
docker: provisioner.Units returns an empty list when app does not
exist.

related to #806.
docker: returning the unit status in provisioner.Units.

related to #806.
docker: getting ip data for provision.Units.

related to #806.
collector: removed code that updates the unit data from provisioner.

related to #806.
provisioner: removing juju.
remove Unit types from app and provisioners.

related to #806.
testing/provision: added method to add a fake unit.

related to #806.
testing: fixed AddUnit behaviour.

AddUnit updates the app.unitLen value.

related to #806.
app: fixed app tests to works with provision.Unit.

* use app.Units() instead app.Unit.
* use AddUnit and Provision to add units instead store it on database.
* clean queue messages on teardown.

related to #806.
provision: fixed tests.

related to #806.
provision/docker: removing saveUnits action and using app.Units().

related to #806.
api: fixed tests.

related to #806.
collector: fixed tests.

related to #806.
testing: fixed FakeApp to implements bind.App.

related to #806.
service: fixed tests.

related to #806.
provision: removed 'Machine' and 'InstanceId' fields from Unit.

related to #806.
provision: added test for Unit.GetIp.

related to 806.
provision: removed app.ProvisionedUnits method.

related to #806.