Allow a profile to be installed from existing config

Problem/Motivation

As a user I want to be able to install Drupal from a package of configuration that is maintained in git.

At DrupalCon New Orleans, a workflow was discussed that would allow a profile would contain the config sync directory and this full config export should be used at install time.

Proposed resolution

If a install profile contains a config/sync directory this will be used to install the site.

To create an install profile that works this way on an existing site you can use https://www.drupal.org/project/install_profile_generator

Alternatively a profile can declare that it can be installed from configuration by defining config_install: true in its info file.
When this is set and the settings.php contains $config_directories[CONFIG_SYNC_DIRECTORY] the profile will be installed from that configuration.

To be clear, this issue would only pertain to certain install profiles. The separate use case of allowing a site to be installed from an existing configuration export which is not part of a profile is covered in #1613424: Allow a site to be installed from existing configuration.

Remaining tasks

Add config validation
Add more test coverage

User interface changes

None

API changes

No API changes per se, there are new steps possible in the installer.

Data model changes

None