Add validation to Escalation Policy Teams: MaxItems 1

[stmcallister]

The PagerDuty API doesn't allow more than one team to be associated with an escalation policy. The provider would run a successful plan but then fail on apply. This change catches that condition during the plan.

Test results:

TF_ACC=1 go test -run "TestAccPagerDutyEscalationPolicy" ./pagerduty -v -timeout 120m
=== RUN   TestAccPagerDutyEscalationPolicy_import
--- PASS: TestAccPagerDutyEscalationPolicy_import (10.53s)
=== RUN   TestAccPagerDutyEscalationPolicy_Basic
--- PASS: TestAccPagerDutyEscalationPolicy_Basic (13.03s)
=== RUN   TestAccPagerDutyEscalationPolicyWithTeams_Basic
--- PASS: TestAccPagerDutyEscalationPolicyWithTeams_Basic (12.50s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	36.604s