cdn: Handle certificate name updates.

[andrewsomething]

This resolves a bug introduced in #500. We made changes to use certificate names as primary identifier instead of IDs. This included the CDN resource, but I missed the update flow for CDNs. We did not have an acceptance test excercising using CDNs with custom domains/certificates.

The new test fails before the code changes are appiled:

$ make testacc TESTARGS='-run=TestAccDigitalOceanCDN_CustomDomain'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccDigitalOceanCDN_CustomDomain -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanCDN_CustomDomain
=== PAUSE TestAccDigitalOceanCDN_CustomDomain
=== CONT  TestAccDigitalOceanCDN_CustomDomain
    resource_digitalocean_cdn_test.go:114: Step 2/2 error: Error running apply: 
        Error: Error deleting Certificate: DELETE https://api.digitalocean.com/v2/certificates/a25f53d6-fdc7-4230-bf25-c6266d48d81a: 422 (request "559bc985-3360-4c4d-a124-71ba0bd83538") This certificate is being used by an active Load Balancer. You must make sure no Load Balancer is using it before deleting.
        
        
--- FAIL: TestAccDigitalOceanCDN_CustomDomain (54.61s)
FAIL
FAIL    github.com/digitalocean/terraform-provider-digitalocean/digitalocean    54.623s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/mutexkv        (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/setutil        (cached) [no tests to run]
FAIL
make: *** [GNUmakefile:16: testacc] Error 1

When the changes are applied, the test now passes:

$ make testacc TESTARGS='-run=TestAccDigitalOceanCDN_CustomDomain'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccDigitalOceanCDN_CustomDomain -timeout 120m
?       github.com/digitalocean/terraform-provider-digitalocean [no test files]
=== RUN   TestAccDigitalOceanCDN_CustomDomain
=== PAUSE TestAccDigitalOceanCDN_CustomDomain
=== CONT  TestAccDigitalOceanCDN_CustomDomain
--- PASS: TestAccDigitalOceanCDN_CustomDomain (95.23s)
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/digitalocean    95.238s
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/datalist       (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/mutexkv        (cached) [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/digitalocean/terraform-provider-digitalocean/internal/setutil        (cached) [no tests to run]

[andrewsomething]

This should fix #578 Though I was not able to recreate the exact same error. I'd like to get a bug fix release out for them to test.

[bentranter]

LGTM!

[liarco]

This should fix #578 Though I was not able to recreate the exact same error. I'd like to get a bug fix release out for them to test.

@andrewsomething I can confirm our bug is fixed with this PR. Thank you very much.