Cluster verify: master must be present for _VerifyFiles

This commit prepares the call to _VerifyFiles for the case when the master
node is not one of the nodes that's being verified (which will be the case
for all node groups but one). We fix it by always passing master info and
checksums to _VerifyFiles, which ensures there's a cluster-wide consistency
check.

Signed-off-by: Adeodato Simo <dato@google.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>