Added instructions for deleting default mysql db to other services

Copied instructions from glance, added to keystone, nova, and heat. Did not
add to neutron, as the chapter layout is significantly different.

Change-Id: If347859a1a38a1353395e4fec58c9f199a72a4eb
backport: havana
Closes-Bug: #1248599
(cherry picked from commit efdf93058c5bd9f66ffe5663f9f9b2b95a74701d)
Merge "Added instructions for deleting default mysql db to other services" into stable/havana