Vm state management and error states

this implements the blueprint nova-vm-state-management
It implements the following functionalities:
- Filter compute api calls according to state of the VM
(defined in compute/state_checker).
- Sets error state if the scheduler cannot allocate the VM in any host
- Handles the create/delete concurrency in the compute manager

Change-Id: Ie6d016b7d4781f70bb5967f204fa88a6412bd727

nova/exception.py

@@ -250,6 +250,11 @@ class InvalidParameterValue(Invalid):
     message = _("%(err)s")
 
 
+class InstanceInvalidState(Invalid):
+    message = _("Instance %(instance_uuid)s in state %(state)s. Cannot "
+                "%(method)s while the instance is in this state.")
+
+
 class InstanceNotRunning(Invalid):
     message = _("Instance %(instance_id)s is not running.")