Migrate sanity_check.py into the respective daemons

ucloud/host/main.py

@@ -71,8 +71,17 @@ def maintenance(host):
             if _vm:
                 running_vms.remove(_vm)
 
+def check():
+    if env_vars.get('STORAGE_BACKEND') == 'filesystem' and not isdir(env_vars.get('VM_DIR')):
+        print("You have set STORAGE_BACKEND to filesystem. So, the vm directory mentioned"
+              " in .env file must exists. But, it don't.")
+        sys.exit(1)
+
+
 
 def main(hostname):
+    check()
+
     heartbeat_updating_process = mp.Process(target=update_heartbeat, args=(hostname,))
 
     host_pool = HostPool(etcd_client, env_vars.get('HOST_PREFIX'))