Drop support for diablo to essex migrations

This removes support for the following legacy-related commands:

    $ keystone-manage export_legacy_catalog
    $ keystone-manage import_legacy
    $ keystone-manage import_nova_auth

DocImpact

Change-Id: If7277e912f11c9bf3bec15f9addd848e0774f14f
Merge "Drop support for diablo to essex migrations"