Prevent timeout for alert_grouping_parameters to be explicitly set to zero when they are of type "content_based"

[cjgajard]

Fixes issue #867

New acceptance test introduced to prevent regression

TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -count=1 -run TestAccPagerDutyService_AlertGroupingParametersAddConfigField -timeout 120m
?       github.com/PagerDuty/terraform-provider-pagerduty       [no test files]
=== RUN   TestAccPagerDutyService_AlertGroupingParametersAddConfigField
--- PASS: TestAccPagerDutyService_AlertGroupingParametersAddConfigField (21.31s)
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerduty     21.978s
testing: warning: no tests to run
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerdutyplugin       1.147s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/util  0.780s [no tests to run]

[imjaroiswebdev]

Great @cjgajard! Thank you addressing this 🎉 💪🏽