cephadm: add a --dry-run option to cephadm shell

cephadm: add a --dry-run option to cephadm shell

Instead of creating the shell, the --dry-run option prints the container
command that would be used. This can be used as a starting point for
creating custom container commands similar to what cephadm shell would
generate but with tweaks.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit db1e785b2c1a55f4d232b1642573e5672a136190)

doc/cephadm: clean up debugging section

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit 4a033788f87859be4da706e38621009474ed3b82)

doc/cephadm: extend container debugging to cover live processes

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit b4e2b3991774fc15f677752a344181920fc1216c)