Validate user-provided configuration is picked up

[sophiewigmore]

The PHP FPM buildpack allows for users to provide their own configuration at <APP ROOT>/.php.ini.d/*.conf. However, the buildpack doesn't log that it found user-provided configuration to include, and we also don't test that when it is included, that it gets picked up.

Issue

We should add two things to ensure this use case is working correctly.

1. We should add (debug?) log lines to the buildpack that show that the buildpack is including user-providing config gile
2. We should add an integration test that includes a user-provided configuration file in the test app. The configuration file can have something like

[www]
pm.max_children=50

When the app starts with a start command such as php-fpm -y $PHP_FPM_PATH -tt, we should assert that pm.max_children = 50 shows up in the output.