Properly force a new resource whenenver the cert is near expiry

[benashz]

Affected resources:

• vault_pki_secret_backend_cert
• vault_pki_secret_backend_sign

The previous approach would leave some computed values out of sync
whenever the certificate determined to be expired. This was
because the approach attempted to leverage a combination of schema CustomizeDiff,
and Update functions, rather than taking advantage of the ability to
mark the resource as requiring replacement within the CustomizeDiff
function itself.

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Closes #835

Release note for CHANGELOG:

Output from acceptance testing:

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

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

[...]

ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
=== RUN   TestPkiSecretBackendCert_basic
--- PASS: TestPkiSecretBackendCert_basic (6.87s)
=== RUN   TestPkiSecretBackendCert_revoke
--- PASS: TestPkiSecretBackendCert_revoke (8.54s)
=== RUN   TestPkiSecretBackendCert_renew
--- PASS: TestPkiSecretBackendCert_renew (11.08s)
=== RUN   TestPkiSecretBackendConfigCA_basic
--- PASS: TestPkiSecretBackendConfigCA_basic (1.71s)
=== RUN   TestPkiSecretBackendConfigUrls_basic
--- PASS: TestPkiSecretBackendConfigUrls_basic (3.16s)
=== RUN   TestPkiSecretBackendCrlConfig_basic
--- PASS: TestPkiSecretBackendCrlConfig_basic (3.87s)
=== RUN   TestPkiSecretBackendIntermediateCertRequest_basic
--- PASS: TestPkiSecretBackendIntermediateCertRequest_basic (1.86s)
=== RUN   TestPkiSecretBackendIntermediateSetSigned_basic
--- PASS: TestPkiSecretBackendIntermediateSetSigned_basic (3.32s)
=== RUN   TestPkiSecretBackendRole_basic
--- PASS: TestPkiSecretBackendRole_basic (3.05s)
=== RUN   TestPkiSecretBackendRootCertificate_basic
--- PASS: TestPkiSecretBackendRootCertificate_basic (10.92s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_default
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_default (2.86s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem (4.25s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_der
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_der (3.53s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle (6.56s)
=== RUN   TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle_multiple_intermediates
--- PASS: TestPkiSecretBackendRootSignIntermediate_basic_pem_bundle_multiple_intermediates (3.28s)
=== RUN   TestPkiSecretBackendSign_basic
--- PASS: TestPkiSecretBackendSign_basic (3.75s)
=== RUN   TestPkiSecretBackendSign_renew
--- PASS: TestPkiSecretBackendSign_renew (13.42s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     (cached)
make testacc TESTARGS='-v -test.run Test*Pki*'  4.74s user 5.53s system 386% cpu 2.657 total


...

[vinay-gopalan]

LGTM!