Fixes #15824 - enabling a repository should fail on pulp error

[johnpmitsch]

When enabling a repository, if there is a pulp error, the task will
be paused after creating the repository in the database, though
it is not created in pulp (since it errored). Moving this to create
in plan phase of Katello::Repository::Create ensures that the task
will stop and not complete on a pulp error, leaving the repository
disabled.

[johnpmitsch]

To test:

1. Don't install ostree
2. Enable an ostree repo in red hat repositories page. (ping me if you need a manifest)
3. ensure it fails with a pulp error The importer type ostree_web_importer does not exist
4. ensure the enable repository task is in the 'stopped' state
5. when you navigate back to the repository page, the ostree repo should be unchecked

[parthaa]

ACK