The CLI help message (--help) doesn't keep the original format

When I create a CLI config option which provides a help text with line
breaks, these line breaks won't get used in the printed help message.

Steps to reproduce
==================
* Create a CLI option which has line breaks in its help text:

    from oslo_config import cfg
    example_opt = cfg.IntOpt('example_opt', default=0,
                             help="""This is an example opt.

    Here is a long description of this example opt which explains this
    more in detail.

    A list which explains things:

    * item 1
    * item 2""")

* Register this CLI opt:

    cfg.register_cli_opt(example_opt, "example_group")

* Let the service which uses this CLI option print its help:

    my-service --help

Expected output
===============

--example_group-example_opt EXAMPLE_GROUP_EXAMPLE_OPT
    This is an example opt.

    Here is a long description of this example opt which explains this
    more in detail.

    A list which explains things:

    * item 1
    * item 2

Actual output
=============

--example_group-example_opt EXAMPLE_GROUP_EXAMPLE_OPT
    This is an example opt. Here is a long description of this example
    opt which explains this more in detail. A list which explains
    things: * item 1 * item 2

Version
=======
* seen in oslo.config version 2.6.0
* NOT double-checked in master code

Notes
=====
I'm using the help extensively in a Nova blueprint [1] and it would be
nice to keep the preformatted text.

References
==========
[1] https://review.openstack.org/#/c/244177/