Install guide: recommend 64-bit OS

Add some text to the install guide to recommend installing a 64-bit
operating system. Also links to Ops Guide for system requirements.

Motivated by this comment in the docs:
http://docs.openstack.org/havana/install-guide/install/apt/content/nova-boot.html#comment-1146233396

Change-Id: Iabe612024be0f6fcf335cf2daaf3ea77bed3cd86
backport: havana
Merge "Install guide: recommend 64-bit OS"