Fix for connecting into Openstack slaves

Currently OpenStack only supports user/password combination for
XenServer, all others need to use ssh with certificates to be able to
login into the new instance for the first time.

See https://wiki.openstack.org/wiki/HypervisorSupportMatrix

This uses the following logic: if you don't specify a password for the
VM and there is a private key set on the JClouds options, it will use
that to connect to the newly created instance.
Merge pull request #44 from cristianonicolai/privatekey

Fix for connecting into Openstack slaves