r/pagerduty_user: Set time_zone as optional

[heimweh]

This PR adds support for managing user time zones,
if not set the PagerDuty account time zone will be used.

[heimweh]

Tests are passing:

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./pagerduty -v  -timeout 120m
=== RUN   TestConfigEmptyToken
--- PASS: TestConfigEmptyToken (0.00s)
=== RUN   TestConfigSkipCredsValidation
2017/07/13 23:45:27 [INFO] PagerDuty client configured
--- PASS: TestConfigSkipCredsValidation (0.00s)
=== RUN   TestAccDataSourcePagerDutyEscalationPolicy_Basic
=== RUN   TestAccDataSourcePagerDutySchedule_Basic
=== RUN   TestAccDataSourcePagerDutyUser_Basic
=== RUN   TestAccDataSourcePagerDutyVendor_Basic
=== RUN   TestAccPagerDutyEscalationPolicy_import
=== RUN   TestAccPagerDutyMaintenanceWindow_import
=== RUN   TestAccPagerDutySchedule_import
=== RUN   TestAccPagerDutyServiceIntegration_import
=== RUN   TestAccPagerDutyService_import
=== RUN   TestAccPagerDutyServiceWithIncidentUrgency_import
=== RUN   TestAccPagerDutyTeamMembership_import
=== RUN   TestAccPagerDutyTeam_import
=== RUN   TestAccPagerDutyUser_import
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProviderImpl
--- PASS: TestProviderImpl (0.00s)
=== RUN   TestAccPagerDutyAddon_Basic
=== RUN   TestAccPagerDutyEscalationPolicy_Basic
=== RUN   TestAccPagerDutyEscalationPolicyWithTeams_Basic
=== RUN   TestAccPagerDutyMaintenanceWindow_Basic
=== RUN   TestAccPagerDutySchedule_Basic
=== RUN   TestAccPagerDutySchedule_BasicWeek
=== RUN   TestAccPagerDutySchedule_Multi
=== RUN   TestAccPagerDutyServiceIntegration_Basic
=== RUN   TestAccPagerDutyServiceIntegrationGeneric_Basic
=== RUN   TestAccPagerDutyService_Basic
=== RUN   TestAccPagerDutyService_BasicWithIncidentUrgencyRules
=== RUN   TestAccPagerDutyService_FromBasicToCustomIncidentUrgencyRules
=== RUN   TestAccPagerDutyTeamMembership_Basic
=== RUN   TestAccPagerDutyTeam_Basic
=== RUN   TestAccPagerDutyUser_Basic
=== RUN   TestAccPagerDutyUserWithTeams_Basic
--- PASS: TestAccPagerDutyTeam_Basic (11.87s)
--- PASS: TestAccPagerDutyTeamMembership_Basic (15.09s)
--- PASS: TestAccPagerDutyUser_Basic (15.59s)
--- PASS: TestAccDataSourcePagerDutyEscalationPolicy_Basic (17.64s)
--- PASS: TestAccPagerDutyService_BasicWithIncidentUrgencyRules (20.75s)
--- PASS: TestAccPagerDutyService_FromBasicToCustomIncidentUrgencyRules (21.68s)
--- PASS: TestAccPagerDutySchedule_Multi (10.31s)
--- PASS: TestAccPagerDutyUserWithTeams_Basic (26.59s)
--- PASS: TestAccPagerDutyServiceIntegrationGeneric_Basic (27.75s)
--- PASS: TestAccPagerDutySchedule_BasicWeek (14.97s)
--- PASS: TestAccPagerDutySchedule_Basic (14.11s)
--- PASS: TestAccPagerDutyUser_import (7.65s)
--- PASS: TestAccPagerDutyServiceIntegration_Basic (23.90s)
--- PASS: TestAccPagerDutyAddon_Basic (9.96s)
--- PASS: TestAccPagerDutyTeam_import (6.01s)
--- PASS: TestAccPagerDutyEscalationPolicyWithTeams_Basic (15.35s)
--- PASS: TestAccPagerDutyEscalationPolicy_Basic (17.68s)
--- PASS: TestAccPagerDutyTeamMembership_import (12.31s)
--- PASS: TestAccPagerDutyMaintenanceWindow_Basic (23.34s)
--- PASS: TestAccPagerDutyServiceWithIncidentUrgency_import (12.25s)
--- PASS: TestAccDataSourcePagerDutyVendor_Basic (4.49s)
--- PASS: TestAccPagerDutyService_import (12.91s)
--- PASS: TestAccPagerDutySchedule_import (12.41s)
--- PASS: TestAccDataSourcePagerDutyUser_Basic (8.04s)
--- PASS: TestAccPagerDutyEscalationPolicy_import (9.65s)
--- PASS: TestAccPagerDutyMaintenanceWindow_import (16.67s)
--- PASS: TestAccPagerDutyServiceIntegration_import (17.61s)
--- PASS: TestAccDataSourcePagerDutySchedule_Basic (10.25s)
--- PASS: TestAccPagerDutyService_Basic (15.89s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	64.462s

[grubernaut]

LGTM, thanks!