Moved uniformity check for exclusive_storage flag

Cluster-verify used to check that the value of exclusive_storage is uniform
within node groups. Now, it's impossible to change the flag for a single
node, so that check has been removed and an equivalent one has been added
in ConfigWriter.VerifyConfig(). Unit test provided. The unit test covers
only the new check, not the whole VerifyConfig method.

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