Support parsing ini files with no sections

Services such as Zookeeper and Kafka use ini files which do not
explicitly specify sections. This change supports merging ini
files with no sections, so that the configuration for these
services follows the example configuration provided for them
as closely as possible.

Closes-Bug: #1756101
Partially-Implements: blueprint monasca-roles
Change-Id: I1061729875e5545c7af7d80779f9c2124b6c7134