Adding pagerduty_priority data source

[stmcallister]

This adds the pagerduty_priority data source and fixes some bugs in the documentation. Addresses issues #216

Test results:

TF_ACC=1 go test -run "TestAccDataSourcePagerDutyPriority_*" ./pagerduty -v -timeout 120m
=== RUN   TestAccDataSourcePagerDutyPriority_Basic
--- PASS: TestAccDataSourcePagerDutyPriority_Basic (2.05s)
=== RUN   TestAccDataSourcePagerDutyPriority_P2
--- PASS: TestAccDataSourcePagerDutyPriority_P2 (2.32s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	5.387s

[heimweh]

Nice job! This LGTM 👍

[ivssh]

Have left a comment regarding the implementation of data source priority in ruleset_rule resource.

[ivssh]

I guess it should be data.pagerduty_priority.p1.id instead of pagerduty_priority.p1.id

[ivssh]

@stmcallister Any thoughts on this?

[pdecat]

FTR, acceptance tests for priorities are failing with development accounts:

=== RUN   TestAccDataSourcePagerDutyPriority_Basic
    data_source_pagerduty_priority_test.go:11: Step 1/1 error: Error running pre-apply refresh: exit status 1

        Error: GET API call to https://api.pagerduty.com/priorities failed: 404 Not Found

          with data.pagerduty_priority.p1,
          on terraform_plugin_test.tf line 2, in data "pagerduty_priority" "p1":
           2: data "pagerduty_priority" "p1" {

--- FAIL: TestAccDataSourcePagerDutyPriority_Basic (1.20s)
=== RUN   TestAccDataSourcePagerDutyPriority_P2
    data_source_pagerduty_priority_test.go:27: Step 1/1 error: Error running pre-apply refresh: exit status 1

        Error: GET API call to https://api.pagerduty.com/priorities failed: 404 Not Found

          with data.pagerduty_priority.p2,
          on terraform_plugin_test.tf line 2, in data "pagerduty_priority" "p2":
           2: data "pagerduty_priority" "p2" {

This test could be skipped if the incident_priority_activated ability is not returned by the https://api.pagerduty.com/abilities endpoint