Move cluster verification out of prepare-node-join

A new tool for configuring the node daemon will also have to verify the
cluster name, so it's better to have this function in a central place.
In the process of moving it to ssconf it is also changed to use
“SimpleStore” instead of reading the ssconf directly. Tests are updated.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>