ceph: output prompt only if stdin is tty

In loop mode, output prompt only if stdin is tty, i.e. if we are
interactive.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1968 from ceph/wip-ceph-isatty

ceph: output prompt only if stdin is tty

Reviewed-by: Sage Weil <sage@inktank.com>