Update config format for replication_devices

Update config format for replication_devices

The first pass at setting up config entries for
replication targets was a hacky custom string parser
with its own unique syntax.

A better option is to use oslo.cfg's MultiOpt in conjunction
with Dict types so we can eliminate quite a bit of custom
parsing and most importantly use a standard syntax for conf
entries to make things easy for admins and reduce the
probability of syntax errors and mistakes in config setttings.

Change-Id: Ie300c1f1db548d258906eebbcea8265583086468
Closes-Bug: #1504696