Add OS verification support to cluster verify

For this, we needed to extend the NodeImage class with a few extra
variables, and we do a trick in the node verification where we pick the
first node that returned valid OS data as the reference node, and then
we compare all other nodes against it.

The checks added are:

- consistency of DiagnoseOS responses
- multiple paths for an OS
- inconsistent OS between a reference node and the current node

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