Fix up some sphinx-isms in nova-manage.rst:
- Replace ``[group]/option`` with :oslo.config:option:`group.option`
- Replace ``[group]`` with :oslo.config:group:`group`
- Replace `default role used for italics` with *explicit italics*
- Fix up a mention of [placement_database] -- for which we can't use
:oslo.config:* because that section doesn't exist anymore -- to
pertain only to Rocky & Stein, where it did exist.
Change-Id: I9cfb9d0b20d4dfc18236a2d528a8f65534c9a263
Eric Fried
9ffdc0bcbc