Added admonition about .ini files

In Heat and Nova sections, where it is recommended to edit an .ini
file, added an admonition about using .ini files general configuration
tasks.

Change-Id: I64cc6edfa9704694961aa80dbb463c707938e381
backport: havana
Closes-bug: #1250304
(cherry picked from commit b6b5fd2b6d38e116961a21cfa037319f1f125451)
Merge "Added admonition about .ini files" into stable/havana