API: Keypair support for X509 public key certificates

Nova keypairs are mostly used by Linux guests to handle user authentication via SSH public key authentication without incurring in the management and security overhead that passwords require.

Public keys are provided to the guests as part of the metadata and included in the guest configuration by tools like cloud-init.

Windows operating systems don't support natively SSH and thus authentication requires the usage of passwords unless the image deployer chooses to include a 3rd party unsupported SSH service port, which implies incurring in potential security and support issues.

Windows supports natively password-less authentication for WinRM by using X509 certificates in place of SSH keys, including Powershell remoting as detailed here: http://www.cloudbase.it/windows-without-passwords-in-openstack/

From a practical perspective, X509 certificates are used by WinRM in a way which can be considered consistent with the usage of SSH keys on Linux, as both are based on public / private keypairs. For simplicity, since WinRM can be configured to accept self signed certificates, we will omit the implications of certificate chain validation.

While Nova currently supports SSH keypairs only, the API can be extended to support x509 certificates, while maintaining full backwards compatibility.

The areas that require changes are:

* Nova API (v2 and v3), to accept the certificate type as an option (defaulting to SSH if not provided)
* The nova.compute.api.KeypairAPI class to be able to generate and manage X509 certificates beside the existing SSH case
* nova.objects.Keypair class to add a type property
* Related database migration
* Nova metadata API to support an additional x509 content in nova.api.metadata.base.InstanceMetadata()
* python-novaclient to add the optional keypair type option

Usage examples

Generating an x509 keypair via CLI and saving the private certificate locally (in PKCS#12 format):

    nova keypair-add keypair1 --keypair-type x509

Importing an x509 public key certificate:

    nova keypair-add keypair1 --keypair-type x509 --pub-key /path/to/PEM_encoded_public_certificate

Note: the certificate type could be discovered by examining the content without requiring the user to provide it explicitly.

For backwards compatibility, omitting the certificate type results in a SSH certificate:

    nova keypair-add keypair2 --pub-key /path/to/SSH_pub_key

equivalent to:

    nova keypair-add keypair2 --keypair-type SSH --pub-key /path/to/SSH_pub_key

On the metadata side ssh keys and x509 keys are provided separately for backwards compatibility:

"public_keys": {"keypair2": "ssh-rsa AAAAB3...cyJvOQ== Generated by Nova\n"},
"x509": {"keypair1": "-----BEGIN CERTIFICATE-----\nMIIDaTC...ysk8w==\n-----END CERTIFICATE-----\n"},

Note: the -----BEGIN CERTIFICATE----- / -----END CERTIFICATE----- header and footer can be omitted