Allow the machines file to specify host public keys

It's easier for the Hydra provisioner to put host public keys in the
machines file than to separately manage the known_hosts file
(especially when the provisioner runs on a different machine).

src/hydra-queue-runner/state.hh

@@ -205,6 +205,7 @@ struct Machine
     std::set<std::string> systemTypes, supportedFeatures, mandatoryFeatures;
     unsigned int maxJobs = 1;
     float speedFactor = 1.0;
+    std::string sshPublicHostKey;
 
     struct State {
         typedef std::shared_ptr<State> ptr;