PXC SST: usability, prepend ',' to sockopt if needed

Currently, we require the user add a comma (",") to the beginning of the option.

This seems like something a user would skip over pretty easily, ending up with some error from socat (which may or may not be helpful).

We should prepend the comma to the sockopt string if it's not there.

Example:
[sst]
sockopt=",cipher=AES128"