Allow installation profiles to partially override module default configuration settings

Follow-up to: #1934700: Automated cron runs should only be enabled by default for Standard profile

Problem

• An installation profile that only wants to override a particular setting from a default module config file needs to override the entire file.
• Otherwise, all other settings are missing.

Goal

• Allow installation profiles to only override what they need to override.

Proposed solution

1. Change the process of installing default configuration files for installation profiles to merge each config file with the module-provided default config file (if any), i.e.:
     modules/system/config/system.cron.yml
+ profiles/standard/config/system.cron.yml
= config/active/system.cron.yml