Made get_capabilities a classmethod instead of object method

Capabilities are the same for all objects so there is no need for them
to be nonstatic. Another benefit is that we can check capabilities of
the Connection without the need to create an object and open real
connection.

Change-Id: Ie9314cb49b4c2bffb2bae4d5eeca6f5d8e2637ce
Merge "Made get_capabilities a classmethod instead of object method"