Rebased PowerDNS provider

[jen20]

This is a rebase of #4181 from @alkersan, adding a bit of formatting cleanup and a slight change to the check destroy method for tests.

With these patches applied:

$ PDNS_SERVER_URL="http://192.168.99.100:8081" PDNS_API_KEY=changeme make testacc TEST=./builtin/providers/powerdns
==> Checking that code complies with gofmt requirements...
go generate ./...
TF_ACC=1 go test ./builtin/providers/powerdns -v  -timeout 120m
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProviderImpl
--- PASS: TestProviderImpl (0.00s)
=== RUN   TestAccPDNSRecord_A
--- PASS: TestAccPDNSRecord_A (0.07s)
=== RUN   TestAccPDNSRecord_AAAA
--- PASS: TestAccPDNSRecord_AAAA (0.06s)
=== RUN   TestAccPDNSRecord_CNAME
--- PASS: TestAccPDNSRecord_CNAME (0.05s)
=== RUN   TestAccPDNSRecord_HINFO
--- PASS: TestAccPDNSRecord_HINFO (0.06s)
=== RUN   TestAccPDNSRecord_LOC
--- PASS: TestAccPDNSRecord_LOC (0.06s)
=== RUN   TestAccPDNSRecord_MX
--- PASS: TestAccPDNSRecord_MX (0.08s)
=== RUN   TestAccPDNSRecord_NAPTR
--- PASS: TestAccPDNSRecord_NAPTR (0.05s)
=== RUN   TestAccPDNSRecord_NS
--- PASS: TestAccPDNSRecord_NS (0.05s)
=== RUN   TestAccPDNSRecord_SPF
--- PASS: TestAccPDNSRecord_SPF (0.05s)
=== RUN   TestAccPDNSRecord_SSHFP
--- PASS: TestAccPDNSRecord_SSHFP (0.04s)
=== RUN   TestAccPDNSRecord_SRV
--- PASS: TestAccPDNSRecord_SRV (0.05s)
=== RUN   TestAccPDNSRecord_TXT
--- PASS: TestAccPDNSRecord_TXT (0.04s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/powerdns   0.665s

[ghost]

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