mgr/cephadm: add a helper function for setting up smb config

mgr/cephadm: add a helper function for setting up smb config

Add a private helper function for copying values into the config_blobs
and similar dictionaries that are serialized and passed to cephadm for
configuration of smb. The helper automates the familiar pattern of
```
if value:
    config_blobs[key] = value
```

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