RunCmd: Support standard input file descriptor

This patch changes “utils.RunCmd” to accept a file-like object or a
numeric file descriptor which will be used as the command's standard
input. One use-case will be to pass all necessary data to
“prepare-node-join”.

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