cephadm: add --dry-run option to cephadm {enter,logs,unit} commands

cephadm: add --dry-run option to cephadm {enter,logs,unit} commands

Add a --dry-run option to the cephadm enter, cephadm logs, and
cephadm unit commands. Like cephadm shell --dry-run, this causes cephadm
to print the command it would have run rather than running said command.
This allows the user to copy and edit or otherwise hack on the output
to make variations on these comands without having to teach cephadm all
the possible options and switches those commands make take.
For instance I can follow recent mgr logging like so:
```
$(/tmp/cephadm logs -i mgr --dry-run | sed 's/ / --since=-1s -f /')
```

Signed-off-by: John Mulligan <jmulligan@redhat.com>