Refs #4611 - db_pending_migration/seed stored in settings

This way, we can determine in the installer if the rake db:migrate/seed
should be run in the installer.

Later, we also could use this information to inform the entering user that
the application is not in ready state yet: preparation for maintenance
page.

Also, rake-ify scripts/foreman-config so that now you can run:

    foreman-rake config -- -k key -v value_to_set

With all the behaviour keeping from the origin script/foreman-config
(the script/foreman-config is still working, although deprecated)
Better? https://github.com/theforeman/foreman/pull/1735/files#diff-40afab022585bfe72f80b963c0981abeR25