Install Guide: Move database section to main neutron.conf

The [database] section has been moved from the plugin files
to the main neutron.conf file for Havana. Follow this change
in all examples.

Also, tag controller with <replaceable> in Neutron chapter.

Change-Id: Ie231bca650e8aac929605238086c645443c5b4ba
backport: havana
Closes-Bug: 1247589
(cherry picked from commit 30b63304e6e70ab7f61707deedac0da97ab7be82)
Merge "Install Guide: Move database section to main neutron.conf" into stable/havana