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

cloudcafe/compute/common/types.py

@@ -127,3 +127,7 @@ class ComputeHypervisors(object):
     KVM = 'kvm'
     QEMU = 'qemu'
     HYPER_V = 'hyper_v'
+
+class InstanceAuthStrategies(object):
+    PASSWORD = 'password'
+    KEY = 'key'