Revert "Only merge nodes that are known to not be offline"

This reverts commit 288f240f.

That commit was buggy at various levels:
  - broke ssh access to the second cluster, making cluster-merge
    unusable (unless ssh key were previously setup?)
  - filtered away offline nodes from being added to the cluster config
    (wrong, they should be kept, as offline)
  - broke commit-check

The previous commit makes the code work again with what this commit
tried to achieve.

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