Add support for importing PKI config URLs

[benashz]

Other fixes:

• Unify create and update funcs
• Test the update case

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Closes #934

Release note for CHANGELOG:

Output from acceptance testing:

 $ time make testacc TESTARGS='-v -test.count 1 -test.run Test*Pki*'           

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -v -test.count 1 -test.run Test*Pki* -timeout 30m ./...

PASS
ok      github.com/hashicorp/terraform-provider-vault/util      0.575s [no tests to run]
=== RUN   TestPkiSecretBackendCert_basic
--- PASS: TestPkiSecretBackendCert_basic (11.21s)
=== RUN   TestPkiSecretBackendCert_renew
--- PASS: TestPkiSecretBackendCert_renew (10.54s)
=== RUN   TestPkiSecretBackendConfigCA_basic
--- PASS: TestPkiSecretBackendConfigCA_basic (1.60s)
=== RUN   TestPkiSecretBackendConfigUrls_basic
--- PASS: TestPkiSecretBackendConfigUrls_basic (3.11s)
=== RUN   TestPkiSecretBackendCrlConfig_basic
--- PASS: TestPkiSecretBackendCrlConfig_basic (3.53s)
=== RUN   TestPkiSecretBackendIntermediateCertRequest_basic
--- PASS: TestPkiSecretBackendIntermediateCertRequest_basic (1.78s)
=== RUN   TestPkiSecretBackendIntermediateSetSigned_basic
--- PASS: TestPkiSecretBackendIntermediateSetSigned_basic (2.94s)
=== RUN   TestPkiSecretBackendRole_basic
--- PASS: TestPkiSecretBackendRole_basic (5.30s)
=== RUN   TestPkiSecretBackendRootCertificate_basic
--- PASS: TestPkiSecretBackendRootCertificate_basic (10.61s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_default
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_default (4.69s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem (2.33s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_der
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_der (3.08s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle (2.92s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle_multiple_intermediates
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle_multiple_intermediates (3.48s)
=== RUN   TestPkiSecretBackendSign_basic
--- PASS: TestPkiSecretBackendSign_basic (4.05s)
=== RUN   TestPkiSecretBackendSign_renew
--- PASS: TestPkiSecretBackendSign_renew (12.09s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     86.141s
make testacc TESTARGS='-v -test.count 1 -test.run Test*Pki*'  75.88s user 30.69s system 114% cpu 1:33.28 total

[vinay-gopalan]

Looks great!