Fix issue when verifying cluster files

If a cluster has any non-master-candidate nodes, those don't contain all
files (e.g. config.data). With commit aef59ae7 (March 31st, 2011)
the logic was changed and subsequently verifying a cluster with non-mc
nodes would complain.

This patch fixes this issue by changing the algorithm. It also adds an
additional check for files which shouldn't exist on a machine. A newly
added unittest is included.

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