PKI: extend revocation support

[benashz]

Other fixes:

• de-duplicate PKI cert revoke tests
• fix typos in PKI docs

Closes #1289

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 ./...

=== RUN   TestPkiSecretBackendCert_basic
--- PASS: TestPkiSecretBackendCert_basic (6.60s)
=== RUN   TestPkiSecretBackendCert_renew
--- PASS: TestPkiSecretBackendCert_renew (16.00s)
=== RUN   TestPkiSecretBackendConfigCA_basic
--- PASS: TestPkiSecretBackendConfigCA_basic (1.63s)
=== RUN   TestPkiSecretBackendConfigUrls_basic
--- PASS: TestPkiSecretBackendConfigUrls_basic (2.81s)
=== RUN   TestPkiSecretBackendCrlConfig_basic
--- PASS: TestPkiSecretBackendCrlConfig_basic (4.28s)
=== RUN   TestPkiSecretBackendIntermediateCertRequest_basic
--- PASS: TestPkiSecretBackendIntermediateCertRequest_basic (1.75s)
=== RUN   TestPkiSecretBackendIntermediateSetSigned_basic
--- PASS: TestPkiSecretBackendIntermediateSetSigned_basic (4.20s)
=== RUN   TestPkiSecretBackendRole_basic
--- PASS: TestPkiSecretBackendRole_basic (2.96s)
=== RUN   TestPkiSecretBackendRootCertificate_basic
--- PASS: TestPkiSecretBackendRootCertificate_basic (7.67s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_default
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_default (4.21s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem (2.91s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_der
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_der (3.59s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle (3.72s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle_multiple_intermediates
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle_multiple_intermediates (5.54s)
=== RUN   TestPkiSecretBackendSign_basic
--- PASS: TestPkiSecretBackendSign_basic (5.13s)
=== RUN   TestPkiSecretBackendSign_renew
--- PASS: TestPkiSecretBackendSign_renew (13.53s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     89.286s
make testacc TESTARGS='-v -test.count 1 -test.run Test*Pki*'  75.92s user 30.32s system 109% cpu 1:37.05 total

[vinay-gopalan]

LGTM!