cephadm/smb: convert config object to use a dataclass

cephadm/smb: convert config object to use a dataclass

While working on the smb.py file I began to be annoyed at the Config
class and the need to repeat myself. Now that cephadm is not expected to
run on python versions older than 3.7 I think it's safe to convert
Config to use a dataclass.

While making the change to a dataclass I also chose to make it a frozen
dataclass to help eliminate any future bugs wrt mutating the config object.

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