cdn: Add test sweeper and retry with backoff.

[andrewsomething]

The CDN API seems to have additional rate limiting applied to it that is stricter than the rest of our API. I also experienced some lag in being able to GET a newly created CDN where it would return 404 for the first few attempts. So this adds a retry with backoff for GETing a CDN.

It also adds a test sweeper and fixes a test failure cause due to the type of certificate that was used.

$ make testacc PKG_NAME=digitalocean/cdn TESTARGS="-count=1"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v ./digitalocean/cdn/... -count=1 -timeout 120m -parallel=2
=== RUN   TestAccDigitalOceanCDN_Create
=== PAUSE TestAccDigitalOceanCDN_Create
=== RUN   TestAccDigitalOceanCDN_Create_with_TTL
=== PAUSE TestAccDigitalOceanCDN_Create_with_TTL
=== RUN   TestAccDigitalOceanCDN_Create_and_Update
=== PAUSE TestAccDigitalOceanCDN_Create_and_Update
=== RUN   TestAccDigitalOceanCDN_CustomDomain
=== PAUSE TestAccDigitalOceanCDN_CustomDomain
=== CONT  TestAccDigitalOceanCDN_Create
=== CONT  TestAccDigitalOceanCDN_Create_and_Update
--- PASS: TestAccDigitalOceanCDN_Create (70.86s)
=== CONT  TestAccDigitalOceanCDN_CustomDomain
--- PASS: TestAccDigitalOceanCDN_Create_and_Update (88.23s)
=== CONT  TestAccDigitalOceanCDN_Create_with_TTL
--- PASS: TestAccDigitalOceanCDN_Create_with_TTL (55.90s)
--- PASS: TestAccDigitalOceanCDN_CustomDomain (89.31s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean/cdn        160.184s

[danaelhe]

🚀