Address name format validation on Escalation Policies

[imjaroiswebdev]

Close #699

Address name format validation on Escalation Policies, following the next pattern "Name can't be blank neither contain '\', '/', '&', '<', '>', nor non-printable characters.", to match API field validation during plan.

Additionally refactor name format validation for Service into a reusable util, which is being used and tested for EP and Service.

New tests cases introduced...

$ make testacc TESTARGS="-run TestAccPagerDutyEscalationPolicy_FormatValidation"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccPagerDutyEscalationPolicy_FormatValidation -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutyEscalationPolicy_FormatValidation
--- PASS: TestAccPagerDutyEscalationPolicy_FormatValidation (0.39s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   1.365s

$ make testacc TESTARGS="-run TestAccPagerDutyService_FormatValidation"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccPagerDutyService_FormatValidation -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutyService_FormatValidation
--- PASS: TestAccPagerDutyService_FormatValidation (0.39s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   0.976s

[drastawi]

@imjaroiswebdev the other use case I saw frequently is the whitespace at the end of a resource name.