Use LU-generated jobs for verifying cluster

This patch moves the logic for verifying the various node groups in a
cluster into the master daemon. Job dependencies are used to ensure the
configuration, which requires the BGL, is verified first.

With this change it will be possible to expose whole-cluster
verification through the remote API without requiring additional client
logic on top of standard features like LU-generated jobs and job
dependencies.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>