Minor: check if promise dir exists before list it

This solve the problem of software release which doesn't provide
any promise

slapos/grid/slapgrid.py

@@ -475,7 +475,8 @@ class Slapgrid(object):
       gid = stat_info.st_gid
 
       # Get the list of promises
-      promise_dir = os.join(instance_path, 'etc', 'promise')
+      promise_dir = os.path.join(instance_path, 'etc', 'promise')
+      if os.path.exists(promise_dir) and os.path.isdir(promise_dir):
         commands_to_run = os.listdir(promise_dir)
         cwd = instance_path