Adding Service Event Rules and Rules Bug Fixes

[stmcallister]

This PR adds the pagerduty_service_event_rule resource, addressing #279.

It also adds suspend and variables fields to pagerduty_ruleset_rule mentioned in #272.

Also allows suppress.value to be set to false. #295, #245, #225.

Fixed bug in pagerduty_ruleset_rule where position wasn't setting properly. The position should now set correctly on the first apply for both pagerduty_ruleset_rule and pagerduty_service_event_rule.

Test results for the pagerduty_service_event_rule resource are here:

TF_ACC=1 go test -run "TestAccPagerDutyServiceEventRule" ./pagerduty -v -timeout 120m
=== RUN   TestAccPagerDutyServiceEventRule_import
--- PASS: TestAccPagerDutyServiceEventRule_import (10.92s)
=== RUN   TestAccPagerDutyServiceEventRule_Basic
--- PASS: TestAccPagerDutyServiceEventRule_Basic (13.19s)
=== RUN   TestAccPagerDutyServiceEventRule_MultipleRules
--- PASS: TestAccPagerDutyServiceEventRule_MultipleRules (14.40s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	39.888s

And for pagerduty_ruleset_rule here:

TF_ACC=1 go test -run "TestAccPagerDutyRulesetRule" ./pagerduty -v -timeout 120m     
=== RUN   TestAccPagerDutyRulesetRule_import
--- PASS: TestAccPagerDutyRulesetRule_import (5.23s)
=== RUN   TestAccPagerDutyRulesetRule_Basic
--- PASS: TestAccPagerDutyRulesetRule_Basic (5.90s)
=== RUN   TestAccPagerDutyRulesetRule_MultipleRules
--- PASS: TestAccPagerDutyRulesetRule_MultipleRules (6.43s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	18.212s

[dobs]

LGTM -- only thing to confirm is whether the tags stuff is intended to be included in this PR as well. Otherwise just two nits.