grid: fix supervisord configuration generation

previous code was matching way too many files: when processing
slappart1, all partitions starting with "slappart1" (e.g. slappart10)
would match too.

This is a fixup of db98a521.

slapos/grid/SlapObject.py

@@ -727,7 +727,7 @@ class Partition(object):
     """
     remaining = set(
       f for f in os.listdir(self.supervisord_partition_configuration_dir)
-      if f.startswith(self.partition_id)
+      if f == '%s.conf' % self.partition_id
     )
     for group, programs in self.supervisor_conf.items():
       filename = '%s.conf' % group