Renew-crypto: rebuild digest map of all nodes

Renew-crypto: rebuild digest map of all nodes

During a renew-crypto operation, all nodes will create
new client certificates. Afterwards, the fingerprints
(digests) of the master candidate nodes needs to be
collected and added to the configuration. This is done
by an RPC call, which will succeed as the master
node's certficate digest was propagated to the nodes
before.

This also removes two unittest which are no longer
necessary, because there will be no RPC call from
the master to itself anymore.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>