gnt-node add: Use prepare-node-join

This patch changes “gnt-node add” to use the newly added
“prepare-node-join” tool. Hereby Paramiko is no longer a hard dependency
for setting up SSH on nodes.

In “gnt_cluster.py”, a positional parameter is no longer passed as a
keyword parameter.

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