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)
They are, I the problem is, after db:migrate, the models are not initialized properly (including the Setting model). I've spent some time on trying to make that working, but it was very hackish and still didn't work well. In the installer, we set the db_pending_migration false right after the migration was run, but for non-installer purposes, we count on fact, that people will run db:migrate and db:seed sequencially, so that they are not affected by this