Add support for several private SSH keys for OpenStack instances
At the moment it is assumed that a user manually creates a key pair with name "ldv" and a public SSH key through, say, a web interface to OpenStack. Then scripts create instances almost referring to this key pair. So there is the only corresponding private SSH key that can be used to access instances.
As a minor issue, it is very welcome to check presence and validness of the private SSH key, since now it can take much time to fall into related errors.