Remove duplicated certificates from chains

[cipherboy]

As reported by Steve Clark, building an intermediate mount in PKI (and
calling /intermediate/set-signed) results in a duplicate intermediate CA
certificate in the full chain output (ca_chain field of the
/cert/ca_chain API endpoint response).

While ideally we'd use proper chain building (and return a sorted
chain), for the time being, return certs in the order they appear and
remove any duplicate certificates.

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

--

This was for the new full chain feature introduced in #13935.

Reproducer:

vault secrets enable -path=pki pki
vault write -field=certificate pki/root/generate/internal \
    common_name=root-kms.com \
    ttl=8760h > /tmp/CA_cert.crt
vault secrets enable -path=pki_int pki
vault write -format=json pki_int/intermediate/generate/internal \
    common_name="example.com Intermediate Authority" \
     | jq -r '.data.csr' > /tmp/pki_intermediate.csr
vault write -format=json pki/root/sign-intermediate csr=@/tmp/pki_intermediate.csr \
     format=pem_bundle ttl="43800h" \
     | jq -r '.data.certificate' > /tmp/intermediate.cert.pem
vault write pki_int/intermediate/set-signed certificate=@/tmp/intermediate.cert.pem
vault read /pki_int/cert/ca_chain

[stevendpclark]

👍