Fixes mis-named parameter in unrescue request

Change-Id: I79067d7eca98776d93105bfa16eff4de3ea4b505
Modifications to allow for instance auth via ssh keys

* Adds private key field to keypair model
* Adds config for instance auth strategy
* Added logic into server behaviors to decide on instance auth strategy

Change-Id: Ia60bc1285bf5d5ba6a86c2a572724ad9d2df7e39