Support to reset instance login key pair

Once the cloud user lost the private key file, and do not have the admin(Linux root) password, they will not be able to login the instance.
Since QEMU guest agent 0.15.0, it has the ability to read/write the guest file. So user can recreate a new keypair, then reset it to the instance.

API:
nova/v2(3)/project_id/servers/server_id -d {"changeKeypair" :{ "keypairName" : "key name" }}

Libvirt Driver,
Potential methods to change the authorized_keys file:
1. QEMU guest agent
2. libguestfs