Fix datasource pagerduty_service_integration not finding service when they're many

[cjgajard]

Fixes #884

$ go test ./... -run TestAccDataSourcePagerDutyServiceIntegration -v
?       github.com/PagerDuty/terraform-provider-pagerduty       [no test files]
?       github.com/PagerDuty/terraform-provider-pagerduty/util/apiutil  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerduty     0.362s [no tests to run]
=== RUN   TestAccDataSourcePagerDutyServiceIntegration_Basic
--- PASS: TestAccDataSourcePagerDutyServiceIntegration_Basic (35.66s)
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerdutyplugin       36.255s
testing: warning: no tests to run
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/util  0.688s [no tests to run]