Install Guide: Clarify database node setup

Some people were not aware that they needed to install MySQL packages
on nodes. This patch rearranges the code so that we have separate
sections for controller and node and thus readers should easily
spot what needs to be done.

Change-Id: I104b949466b426e102d55bcb3c4a9cb72a0a6af9
Closes-Bug: #1250600
backport: havana
Merge "Install Guide: Clarify database node setup"