Install guide: don't duplicate AMQP configuration

The nova-network section redefines the rabbit_host option. This doesn't
hurt but might lead to problems if the user wants to change the settings
later.

Change-Id: I71481813c77dfa40ca664e99afe7885141b6361f
backport: havana
(cherry picked from commit 510d0901ea5161a5e562334275399b1ec7b6509e)
Merge "Install guide: don't duplicate AMQP configuration" into stable/havana