resource/cloudflare_zone: support setting type

[jacobbednarz]

Updates the cloudflare_zone resource to support setting the zone type
when creating and updating the resource.

Fixes #1277

[jacobbednarz]

acceptance tests are all green

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./...) -v -run "^TestAccCloudflareZone_" -count 1 -parallel 1 -timeout 120m -parallel 1
?   	github.com/cloudflare/terraform-provider-cloudflare	[no test files]
=== RUN   TestAccCloudflareZone_PreventZoneIdAndNameConflicts
=== PAUSE TestAccCloudflareZone_PreventZoneIdAndNameConflicts
=== RUN   TestAccCloudflareZone_NameLookup
=== PAUSE TestAccCloudflareZone_NameLookup
=== RUN   TestAccCloudflareZone_Basic
--- PASS: TestAccCloudflareZone_Basic (11.37s)
=== RUN   TestAccCloudflareZone_BasicWithJumpStartEnabled
--- PASS: TestAccCloudflareZone_BasicWithJumpStartEnabled (11.55s)
=== RUN   TestAccCloudflareZone_WithPlan
--- PASS: TestAccCloudflareZone_WithPlan (12.29s)
=== RUN   TestAccCloudflareZone_PartialSetup
--- PASS: TestAccCloudflareZone_PartialSetup (11.87s)
=== RUN   TestAccCloudflareZone_FullSetup
--- PASS: TestAccCloudflareZone_FullSetup (12.69s)
=== RUN   TestAccCloudflareZone_WithEnterprisePlan
--- PASS: TestAccCloudflareZone_WithEnterprisePlan (16.94s)
=== RUN   TestAccCloudflareZone_SetType
--- PASS: TestAccCloudflareZone_SetType (24.60s)
=== CONT  TestAccCloudflareZone_PreventZoneIdAndNameConflicts
--- PASS: TestAccCloudflareZone_PreventZoneIdAndNameConflicts (2.74s)
=== CONT  TestAccCloudflareZone_NameLookup
--- PASS: TestAccCloudflareZone_NameLookup (10.68s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/cloudflare	115.014s
?   	github.com/cloudflare/terraform-provider-cloudflare/version	[no test files]