integration_pagerduty: fix perpetual diff in api_token

[bendrucker]

This PR fixes a perpetual diff observed in datadog_integration_pagerduty.api_token, when this value is unset:

- api_token           = (sensitive value)

I called the API and realized that it is returning a redacted string even though this is unset:

$ https get https://api.datadoghq.com/api/v1/integration/pagerduty "DD-API-KEY: ${DATADOG_API_KEY}" "DD-APPLICATION-KEY: ${DATADOG_APP_KEY}"
HTTP/1.1 200 OK
Cache-Control: no-cache
Connection: keep-alive
Content-Encoding: gzip
Content-Security-Policy: frame-ancestors 'self'; report-uri https://api.datadoghq.com/csp-report
Content-Type: application/json
DD-POOL: dogweb
Date: Fri, 22 May 2020 19:44:14 GMT
Pragma: no-cache
Set-Cookie: DD-PSHARD=113; Max-Age=604800; Path=/; expires=Fri, 29-May-2020 19:44:14 GMT; secure; HttpOnly
Strict-Transport-Security: max-age=15724800;
Transfer-Encoding: chunked
Vary: Accept-Encoding
X-Content-Type-Options: nosniff
X-DD-Debug: PKDIrz8Hcluof9oNzY3q1BouPTowe6nlZ4slm6KLsMEc/9DaK1hteKVCh6mza/IQ
X-DD-VERSION: 35.2535746
X-Frame-Options: SAMEORIGIN

{
    "api_token": "*****",
    "schedules": [],
    "services": [],
    "subdomain": "foo"
}

So when Read persists this value to state, Terraform believes that it needs to remove a remotely set value.

Instead, because a real value is never returned, this attribute should not be set in Read. This means that Terraform cannot detect drift, but if a user updates the value Terraform will still call Update.

Acceptance test output:

$ make cassettes TESTARGS='-run TestAccDatadogIntegrationPagerduty'
==> Checking that code complies with gofmt requirements...
RECORD=true TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccDatadogIntegrationPagerduty -timeout 120m
?   	github.com/terraform-providers/terraform-provider-datadog	[no test files]
=== RUN   TestAccDatadogIntegrationPagerdutyServiceObject_Basic
--- PASS: TestAccDatadogIntegrationPagerdutyServiceObject_Basic (7.92s)
=== RUN   TestAccDatadogIntegrationPagerduty_Basic
--- PASS: TestAccDatadogIntegrationPagerduty_Basic (1.90s)
=== RUN   TestAccDatadogIntegrationPagerduty_TwoServices
--- PASS: TestAccDatadogIntegrationPagerduty_TwoServices (1.87s)
=== RUN   TestAccDatadogIntegrationPagerduty_Migrate2ServiceObjects
--- PASS: TestAccDatadogIntegrationPagerduty_Migrate2ServiceObjects (6.00s)
PASS
ok  	github.com/terraform-providers/terraform-provider-datadog/datadog	18.205s
?   	github.com/terraform-providers/terraform-provider-datadog/version	[no test files]

[gzussa]

Good catch and it seems legit. Thank a lot