Fix the confusing ssh/hostname message in node add

Before, it used to say:

  ssh/hostname verification failed node1.example.com -> hostname mismatch, got
  node2

Now it says for wrong hostnames (maybe too verbose):

  ssh/hostname verification failed (checking from node1.example.com): hostname
  mismatch, expected node2.example.com but got node3

And for non-FQDN hostnames:

  ssh/hostname verification failed (checking from node1.example.com): hostname
  not FQDN: expected node2.example.com but got node2

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