Add asymmetric key generation in dogtag plugin

This patch adds methods to support asymmetric key generation using
the dogtag plugin. The supported algorithms are RSA and DSA.
Corresponding tests have been added to test_dogtag.py.

Change-Id: Ic8a7635ce98f3220f85740085072d73f66282cd1
Merge "Add asymmetric key generation in dogtag plugin"