resource/logpush_job: Add support for import

[jacobbednarz]

Updates the logpush job to support the Import method.

Closes #616

[jacobbednarz]

Note, we can't add an integration test for this Import due to not being able to mock the ownership_challenge in tests (as we've previously discovered in #335).

[jamesarosen]

Success!

$ terraform import cloudflare_logpush_job.example-com myzoneid/12345
cloudflare_logpush_job.example-com: Importing from ID "myzoneid/12345"...
cloudflare_logpush_job.example-com: Import prepared!
  Prepared cloudflare_logpush_job for import
cloudflare_logpush_job.example-com: Refreshing state... [id=12345]

Import successful!

The resources that were imported are shown above. These resources are now in
your Terraform state and will henceforth be managed by Terraform.