Fix Azure attestation validation

[hslatman]

Fixes #11

Example using OpenSSL:

$ cat att.p7.pem
-----BEGIN PKCS7-----
MIIEEgYJKoZIhvcNAQcCoIIEAzCCA/8CAQExDzANBgkqhkiG9w0BAQsFADCBugYJ
KoZIhvcNAQcBoIGsBIGpeyJub25jZSI6IjEyMzQ1NjY3NjYiLCJwbGFuIjp7Im5h
bWUiOiIiLCJwcm9kdWN0IjoiIiwicHVibGlzaGVyIjoiIn0sInRpbWVTdGFtcCI6
eyJjcmVhdGVkT24iOiIxMS8yMC8xOCAyMjowNzozOSAtMDAwMCIsImV4cGlyZXNP
biI6IjExLzIwLzE4IDIyOjA4OjI0IC0wMDAwIn0sInZtSWQiOiIifaCCAj8wggI7
MIIBpKADAgECAhBnxW5Kh8dslEBA0E2mIBJ0MA0GCSqGSIb3DQEBBAUAMCsxKTAn
BgNVBAMTIHRlc3RzdWJkb21haW4ubWV0YWRhdGEuYXp1cmUuY29tMB4XDTE4MTEy
MDIxNTc1N1oXDTE4MTIyMDIxNTc1NlowKzEpMCcGA1UEAxMgdGVzdHN1YmRvbWFp
bi5tZXRhZGF0YS5henVyZS5jb20wgZ8wDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGB
AML/tBo86ENWPzmXZ0kPkX5dY5QZ150mA8lommszE71x2sCLonzv4/UWk4H+jMMW
RRwIea2CuQ5RhdWAHvKq6if4okKNt66fxm+YTVz9z0CTfCLmLT+nsdfOAsG1xZpp
EapC0Cd9vD6NCKyE8aYI1pliaeOnFjG0WvMY04uWz2MdAgMBAAGjYDBeMFwGA1Ud
AQRVMFOAENnYkHLa04Ut4Mpt7TkJFfyhLTArMSkwJwYDVQQDEyB0ZXN0c3ViZG9t
YWluLm1ldGFkYXRhLmF6dXJlLmNvbYIQZ8VuSofHbJRAQNBNpiASdDANBgkqhkiG
9w0BAQQFAAOBgQCLSM6aX5Bs1KHCJp4VQtxZPzXF71rVKCocHy3N9PTJQ9Fpnd+b
Yw2vSpQHg/AiG82WuDFpPReJvr7Pa938mZqW9HUOGjQKK2FYDTg6fXD8pkPdyghl
X5boGWAMMrf7bFkup+lsT+n2tRw2wbNknO1tQ0wICtqy2VqzWwLi45RBwTGB6DCB
5QIBATA/MCsxKTAnBgNVBAMTIHRlc3RzdWJkb21haW4ubWV0YWRhdGEuYXp1cmUu
Y29tAhBnxW5Kh8dslEBA0E2mIBJ0MA0GCSqGSIb3DQEBCwUAMA0GCSqGSIb3DQEB
AQUABIGAld1BM/yYIqqv8SDE4kjQo3Ul/IKAVR8ETKcve5BAdGSNkTUooUGVniTX
euvDj5NkmazOaKZp9fEtByqqPOyw/nlXaZgOO44HDGiPUJ90xVYmfeK6p9RpJBu6
kiKhnnYTelUk5u75phe5ZbMZfBhuPhXmYAdjc7Nmw97nx8NnprQ=
-----END PKCS7-----

$ openssl pkcs7 -inform der -in att.p7.der -print_certs
subject=CN = testsubdomain.metadata.azure.com
issuer=CN = testsubdomain.metadata.azure.com
-----BEGIN CERTIFICATE-----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=
-----END CERTIFICATE-----

$ openssl pkcs7 -inform der -in att.p7.der -print_certs -out att.p7.crt

$ openssl cms -verify -binary -inform der -in att.p7.der -certfile att.p7.crt -noverify -nointern
CMS Verification successful
{"nonce":"1234566766","plan":{"name":"","product":"","publisher":""},"timeStamp":{"createdOn":"11/20/18 22:07:39 -0000","expiresOn":"11/20/18 22:08:24 -0000"},"vmId":""}%

# shorter:
$ openssl cms -verify -binary -inform der -in att.p7.der -noverify
CMS Verification successful
{"nonce":"1234566766","plan":{"name":"","product":"","publisher":""},"timeStamp":{"createdOn":"11/20/18 22:07:39 -0000","expiresOn":"11/20/18 22:08:24 -0000"},"vmId":""}%