python-karborclient: get() method on a checkpoint object fails

Each resource (plan, provider, checkpoint, etc.) in python-karborclient inherits from Resource, which has a `.get()` method to update the resource attributes.

The `.get()` method on a Checkpoint object fails:

    File "karborclient/common/base.py", line 325, in get
        new = self.manager.get(self.id)
    TypeError: get() takes at least 3 arguments (2 given)

This is caused because Resource `.get()` sends the get request to the manager with the resource (in this case, a Checkpoint) id, and without the provider id required for the checkpoint get.

Issue easily reproduced when trying to call `.get()` on a Checkpoint object created with python-karborclient