Install Guide: Clarify MySQL instructions

The previous wording confused users, it was not clear that the database
server packages need to be installed. I hope that this change is now
clearer.

Change-Id: I5ddf3848988205c284c4044b32f471178ad29947
Closes-Bug: #1254662
backport: havana
(cherry picked from commit b07188669e7a8e56c42bb2d94d5589a0cfb37b8b)
Merge "Install Guide: Clarify MySQL instructions" into stable/havana