Warnings about custom config are not strictly correct - improve them

[rfay]

What happened (or feature request):

PHP config override must be .ini files, and mysql config override must be .cnf files. However, we notify about all files that exist in the relevant directories. We should only warn about .ini and .cnf.

Starting environment for d8git...
Using custom mysql configuration: [junk.cnf.broken]
Custom configuration takes effect when container is created,
usually on start, use 'ddev restart' if you're not seeing it take effect.

What you expected to happen:

How to reproduce this:

Version: Please include the output of ddev version, docker version and the project's .ddev/config.yaml.

Anything else do we need to know:

Related source links or issues:

Please use a complexity rating of 1-5 (5 is high) for a feature request. (High complexity implies more PR planning)