cli: Keep SSH connection alive more often

Commit d853eeb ("cli: Keep SSH connection alive") enabled an ssh
keepalive ping every 30 seconds to stop eager network applicances for
tearing down the connection. This solved the issues we were previously
seeing but I think I saw something similar recently. So let's increase
it to a 15 second ping to be safe.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>