Update the way that the certificate bundle is generated

[benashz]

De-duplicate all certs from a PEM bundle by first decoding all blocks.

• update all related unit tests so that they are passing again.

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Relates OR Closes #0000

Release note for CHANGELOG:

Output from acceptance testing:

$ make testacc TESTARGS='-v -test.run TestPkiSecretBackendCert*'                

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

ok      github.com/hashicorp/terraform-provider-vault/util      0.473s [no tests to run]
=== RUN   TestPkiSecretBackendCert_basic
--- PASS: TestPkiSecretBackendCert_basic (5.80s)
=== RUN   TestPkiSecretBackendCert_revoke
--- PASS: TestPkiSecretBackendCert_revoke (5.60s)
=== RUN   TestPkiSecretBackendCert_renew
--- PASS: TestPkiSecretBackendCert_renew (11.81s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     24.317s

=== RUN   Test_setCAChain
=== RUN   Test_setCAChain/empty-ca-chain-pem
=== RUN   Test_setCAChain/empty-ca-chain-pem-bundle
=== RUN   Test_setCAChain/empty-ca-chain-2-pem
=== RUN   Test_setCAChain/empty-ca-chain-2-duplicate-pem
=== RUN   Test_setCAChain/empty-ca-chain-2-pem-bundle
=== RUN   Test_setCAChain/empty-ca-chain-2-duplicate-pem-bundle
=== RUN   Test_setCAChain/empty-ca-chain-der
=== RUN   Test_setCAChain/absent-ca-chain-der
=== RUN   Test_setCAChain/populated-ca-chain
=== RUN   Test_setCAChain/invalid-ca-chain-type
=== RUN   Test_setCAChain/missing-intermediate-cert
=== RUN   Test_setCAChain/missing-issuing-ca
--- PASS: Test_setCAChain (0.00s)
    --- PASS: Test_setCAChain/empty-ca-chain-pem (0.00s)
    --- PASS: Test_setCAChain/empty-ca-chain-pem-bundle (0.00s)
    --- PASS: Test_setCAChain/empty-ca-chain-2-pem (0.00s)
    --- PASS: Test_setCAChain/empty-ca-chain-2-duplicate-pem (0.00s)
    --- PASS: Test_setCAChain/empty-ca-chain-2-pem-bundle (0.00s)
    --- PASS: Test_setCAChain/empty-ca-chain-2-duplicate-pem-bundle (0.00s)
    --- PASS: Test_setCAChain/empty-ca-chain-der (0.00s)
    --- PASS: Test_setCAChain/absent-ca-chain-der (0.00s)
    --- PASS: Test_setCAChain/populated-ca-chain (0.00s)
    --- PASS: Test_setCAChain/invalid-ca-chain-type (0.00s)
    --- PASS: Test_setCAChain/missing-intermediate-cert (0.00s)
    --- PASS: Test_setCAChain/missing-issuing-ca (0.00s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     0.670s

=== RUN   Test_pkiSecretRootSignIntermediateRUpgradeV0
=== RUN   Test_pkiSecretRootSignIntermediateRUpgradeV0/basic
=== RUN   Test_pkiSecretRootSignIntermediateRUpgradeV0/invalid-no-issuing-ca
=== RUN   Test_pkiSecretRootSignIntermediateRUpgradeV0/invalid-no-certificate
--- PASS: Test_pkiSecretRootSignIntermediateRUpgradeV0 (0.00s)
    --- PASS: Test_pkiSecretRootSignIntermediateRUpgradeV0/basic (0.00s)
    --- PASS: Test_pkiSecretRootSignIntermediateRUpgradeV0/invalid-no-issuing-ca (0.00s)
    --- PASS: Test_pkiSecretRootSignIntermediateRUpgradeV0/invalid-no-certificate (0.00s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     1.732s

[vinay-gopalan]

Looks great!