Engine: Organization: update so that katello org is an extension of the core org.

This PR contains the changes needed to make the katello organization an extension of the core (foreman) organization.  The approach taken is very similar to what was done to support a single unified user.

The PR addresses only the changes to support this from a model/schema  perspective.  Future PRs will address the changes needed in the UI.

A couple of things to be aware of:
- schema/migration:  This PR deletes the 'katello_organizations' & 'katello_organization_users' from the migrations and adds additional attributes to the core organization.  Since we are very early in the engine work and in order to minimize complexity, the data from those tables is not being migrated to the core organization.  As a result, developers will need to katello:reset after this PR is merged.
- from a testing point of view, the tests will now use the taxonomies (organization) fixture for the core when working with organizations (e.g. :organization1,:organization2).
- the seeds.rb will create an ACME_Corporation organization (as  it did before).  You should now see that organization in both the foreman and katello UI.  In addition, you should still be able to use those UIs to edit the organization.