21678: Passes credentials through conf file instead of env vars.

21678: Passes credentials through conf file instead of env vars.

To avoid leaking the token to the remote host process list, and also
minimize the exposure on the local host, instead of passing the credentials
through environment variables, we build a conf file that get piped to ssh
via stdout/stdin.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas.dipentima@curii.com>