Starting from RabbitMQ 3.7.0 it's recommended to use new-style
config which is simply an ini file.
It's easier to read and maintain config file in ini fromat rather then
in classic erlang.
At the same time we still keep old-style config as it might have settings
that are not supported in new-style config.
There're no evidences that used there options are still supported,
but it's worth deprecating them in follow-up patch anyway.
Change-Id: I239366ad4aa2bc7a02d826b6c2f94631f4b0e622
e707eecdd8