Add a function to validate an x509 RSA key pair

Add a function to validate an x509 RSA certificate and key pair, as
commonly used for TLS certificates.

The rationale behind this is that we store our TLS certificates and
private keys in Hiera YAML files, and poor indentation or formatting in
the YAML file could cause a valid certificate to be considered invalid.

Will cause the Puppet run to fail if:

- an invalid certificate is detected
- an invalid RSA key is detected
- the certificate does not match the key, i.e. the certificate
  has not been signed by the supplied key

The test certificates I've used in the spec tests were generated using
the Go standard library:

    $ go run $GOROOT/src/crypto/tls/generate_cert.go -host localhost

Example output:

    ==> cache-1.router: Error: Not a valid RSA key: Neither PUB key nor PRIV key:: nested asn1 error at /var/govuk/puppet/modules/nginx/manifests/config/ssl.pp:30 on node cache-1.router.dev.gov.uk

1 files changed, 16 insertions, 0 deletions

diff --git a/README.markdown b/README.markdown
index 9ff242c..e126e5d 100644
--- a/README.markdown
+++ b/README.markdown
@@ -1182,6 +1182,22 @@ Instead, use:
 
 *Type*: statement.
 
+#### `validate_x509_rsa_key_pair`
+
+Validates a PEM-formatted X.509 certificate and private key using OpenSSL.
+Verifies that the certficate's signature was created from the supplied key.
+
+Fails catalog compilation if any value fails this check.
+
+Takes two arguments, the first argument must be a X.509 certificate and the
+second must be an RSA private key:
+
+  ~~~
+  validate_x509_rsa_key_pair($cert, $key)
+  ~~~
+
+*Type*: statement.
+
 #### `values`
 
 Returns the values of a given hash. For example, given `$hash = {'a'=1, 'b'=2, 'c'=3} values($hash)` returns [1,2,3].