adding retry to user update

[stmcallister]

This adds retry logic to the user update function.

TF_ACC=1 go test -run "TestAccPagerDutyUser" ./pagerduty -v -timeout 120m
=== RUN   TestAccPagerDutyUserContactMethod_import
--- PASS: TestAccPagerDutyUserContactMethod_import (6.60s)
=== RUN   TestAccPagerDutyUserNotificationRule_import
--- PASS: TestAccPagerDutyUserNotificationRule_import (8.26s)
=== RUN   TestAccPagerDutyUser_import
--- PASS: TestAccPagerDutyUser_import (5.32s)
=== RUN   TestAccPagerDutyUserContactMethodEmail_Basic
--- PASS: TestAccPagerDutyUserContactMethodEmail_Basic (7.41s)
=== RUN   TestAccPagerDutyUserContactMethodPhone_Basic
--- PASS: TestAccPagerDutyUserContactMethodPhone_Basic (8.31s)
=== RUN   TestAccPagerDutyUserContactMethodSMS_Basic
--- PASS: TestAccPagerDutyUserContactMethodSMS_Basic (8.78s)
=== RUN   TestAccPagerDutyUserNotificationRuleContactMethod_Basic
--- PASS: TestAccPagerDutyUserNotificationRuleContactMethod_Basic (12.18s)
=== RUN   TestAccPagerDutyUser_Basic
--- PASS: TestAccPagerDutyUser_Basic (6.80s)
=== RUN   TestAccPagerDutyUserWithTeams_Basic
--- PASS: TestAccPagerDutyUserWithTeams_Basic (10.77s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	75.137s

[heimweh]

Nice work! 🙂 This LGTM 👌