The InfluxDB config file is an exotic format containing nested
sections, for example ```[[foo]]```, and doesn't work with
merge_configs. To support customising the config this patch adds
support for overriding at the file level.
Change-Id: I05b18c43c3a18e9b46dc450565bc1e156b4ff9dd
Closes-Bug: #1757186
Partially-Implements: blueprint monasca-roles
e7da510cd6