mgr/ssh: pipe ceph-daemon script to stdin of python3

mgr/ssh: pipe ceph-daemon script to stdin of python3

This avoids any need for the script to be present on the remote host.

We introduce a config option to indicate where the script should be
read from, since the location varies between a vstart environment (source
dir) and a real install (/usr/sbin).

Signed-off-by: Sage Weil <sage@redhat.com>