Add command "app:config:status" to check if "app:config:import" needed

[jalogut]

Description

Add command app:config:status to check whether config propagation is up to date. This command can be used to decide whether app:config:import execution is needed during deployments.

Fixed Issues

On version 2.2 a new command setup:db:status was added to decide whether executing setup:upgrade is required. The purpose of this command was to add the possibility of deploying without executing setup:upgrade if module versions are up to date.

With the same objective, a command to check the config status is needed to decide whether app:config:import is needed.

Issue
If setup:upgrade is not executed, we need to always execute app:config:import because there is no way to know, if config files are up to date or not. This will mean, we still need to set the maintenance and we cannot simply switch the code to the new version.

Solution
Using app:config:status, we would be able to switch to a new version without the need of a maintenance, if setup:db:status also returns that everything is up to date

Manual testing scenarios

Thanks to this command, we would be able to deploy without maintenance as follows:

1.- Check setup:db:status returns All modules up to date
2.- Check app:config:status returns Configuration files are up to date.
3.- No maintenance is required because we do not need to execute setup:upgrade or app:config:import

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[jalogut]

This PR adds improvements in same area as:

• Add argument on app:config:dump to skip dumping all system settings. #12410
• Making possible to define configuration by environment #12361
• Add option "lock-config" for shell command "config:set" #12178

[hostep]

Quick heads-up: This was supposed to be added to Magento 2.2.4, and it is, but unfortunately the line which was added to the di.xml file has been (accidentally?) removed by this commit: e6b6c65#r28828688

Can someone try to figure out how this happened and if something needs to happen around this? In Magento 2.2.4 the line is gone, in the 2.2-develop branch it still exists (at the moment of writing) ...