Move optional configuration in config/install to config/optional

I love this change - reading the code in the configuration installer it's so much easier to understand. Personally I think we should roll this work into #2090115: Don't install a module when its default configuration has unmet dependencies since it solves the problem of unmet dependencies and optional configuration completely. That said, I'm posting patches on this issue to get it green before going back to that issue. So no need for reviews on this issue at this point.

Used a PHP 5.6 only thing :(

This should fix all the tests.

• For the webtest fails we have modules which did not correctly declare their dependencies - for example block_test is not dependent on block.
• For kerneltest fails we need to install the configuration for the thing we're testing rather than relying on the the magic find all the configuration in already install modules.
• The KernelTestBaseTest failure were interesting because module_test depends on node which depends on user which depends on system. The user module needs the dependency on system because it provides system.action. configuration entities and the kernel test base environment is special because system is not installed unless you tell it to be :)

One more thing...

Moved patch to #2090115: Don't install a module when its default configuration has unmet dependencies

This became part of #2090115: Don't install a module when its default configuration has unmet dependencies