Improvements for escalation policies

[cjgajard]

• Prevent null-pointer panic at escalation_policy
• Fix broken tests for escalation policies
• Use pagination for data.pagerduty_escalation_policy (Fixes #877 )

=== RUN   TestAccPagerDutyEscalationPolicy_import
--- PASS: TestAccPagerDutyEscalationPolicy_import (12.77s)
=== RUN   TestAccPagerDutyEscalationPolicy_Basic
--- PASS: TestAccPagerDutyEscalationPolicy_Basic (25.84s)
=== RUN   TestAccPagerDutyEscalationPolicy_FormatValidation
--- PASS: TestAccPagerDutyEscalationPolicy_FormatValidation (0.57s)
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerduty     40.186s

=== RUN   TestAccDataSourcePagerDutyEscalationPolicy_Basic
--- PASS: TestAccDataSourcePagerDutyEscalationPolicy_Basic (13.98s)
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerduty     14.891s