svcbackend: use a shorter name for supervisor socket

When we run tests of slaprunner we have:

  >>> len('/srv/slapgrid/slappart14/srv/testnode/cpa/inst/test0-0/tmp/inst/s0/srv/runner/instance/supervisord.socket')
  105

This socket is never supposed to be used directly, so we can use a
shorter path.

slapos/grid/svcbackend.py

@@ -65,7 +65,9 @@ def getSupervisorRPC(socket):
       yield s.supervisor
 
 def _getSupervisordSocketPath(instance_root):
-  return os.path.join(instance_root, 'supervisord.socket')
+  # This needs to be short, because linux has a very low limit
+  # for the maximmal path length of sockets.
+  return os.path.join(instance_root, 'sv.sock')
 
 def _getSupervisordConfigurationFilePath(instance_root):
   return os.path.join(instance_root, 'etc', 'supervisord.conf')