Fix driver initialization with config properties for the right method (T1784)

Summary:
Proposed fix for the bug reported in T1784

The method kolab_2fa::get_driver() is called with the full factor ID (<method>:<uuid>)
and therefore fails to retrieve the right properties from plugin config. This change fixes this at the right place.

Reviewers: machniak

Reviewed By: machniak

Subscribers: jh23453

Differential Revision: https://git.kolab.org/D242