provider/openstack: Add openstack_dns_zone_v2 resource

[jtopjian]

This commit adds the openstack_dns_zone_v2 resource to manage a DNS zone in the OpenStack DNS service.

For #14603

[jtopjian]

==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/22 01:48:36 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/openstack -v -run=TestAccDNSV2Zone -timeout 120m
=== RUN   TestAccDNSV2Zone_importBasic
--- PASS: TestAccDNSV2Zone_importBasic (35.67s)
=== RUN   TestAccDNSV2Zone_basic
--- PASS: TestAccDNSV2Zone_basic (61.08s)
=== RUN   TestAccDNSV2Zone_readTTL
--- PASS: TestAccDNSV2Zone_readTTL (34.94s)
=== RUN   TestAccDNSV2Zone_timeout
--- PASS: TestAccDNSV2Zone_timeout (35.09s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/openstack      166.814s

@stack72 This is all good on my side. Since I have commits in this PR, can you do a quick code review?

[stack72]

LGTM sir :) Merge at will :)

[ghost]

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