Doc: Clusters from Scratch: Improve SSH config

The dsa key in the doc previously did not allow passwordless login (as
the "test" section showed). Support was disabled by default as of
OpenSSH v7.0.

• https://www.openssh.com/txt/release-7.0

See also:

• https://superuser.com/a/1016990

Other updates to the SSH config section:

• Append to authorized_keys instead of overwriting.
• Use ssh-copy-id to copy key to other node.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>