promise: use '==' instead of 'is' to compare two strings

This solve indefinite loop in while condition on promise check_free_disk_space

slapos/promise/plugin/check_free_disk_space.py

@@ -109,7 +109,7 @@ class RunPromise(GenericPromise):
       else:
         break
       for p in partitions:
-        if p.mountpoint is path:
+        if p.mountpoint == path:
           disk_partition = p.device
           break
     if not disk_partition: