mgr/ssh: simplify ssh connection management

mgr/ssh: simplify ssh connection management

- Use a single instance of the config and identity files for the whole
module.  There's no need to create these for *every* connection--it just
pollutes /tmp.
- Drop the SSHConnection wrapper, since the temp files are tied to the
daemon lifecycle now.
- Prefix the tmp files so I can tell wtf is going on.
- Always connect to root@host, to avoid remoto's localhost detection
feature.  This ensures we have a consistent connection model and user.
(The daemon might be running as user ceph and try to connect to localhost,
but end up running the command as the wrong user and/or inside the
ceph-mgr container.)

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