Fix drbd driver to load without 3'rd party libs

Fix drbd driver to load without 3'rd party libs

The new drbd driver has a couple needed 3'rd party libs
that aren't in the global requirements.  That means that
any time there's an attempt to import the driver things
blow up due to import errors.

The great thing is that the author took care of things in
unit tests by creating mocks, but we still have the problem
of the driver itself, and this causes us to be unable to do
things like genconfig.

This patch just wraps the 3'rd party libs into a try/catch
block, sets them to None if they're not on the system.

Also, move the global dbus.array variable into init and
make it a member variable.

Change-Id: I33b605fecf1709efa6124312ba58cd29ed885178
Closes-Bug: #1408483