ssh.py: use 'exec python -c' instead of just 'python -c'.

This gets rid of an extra intermediate sh process on the server that we were
keeping for no good reason, since it would exit as soon as python exited
anyway.

ssh.py

@@ -85,7 +85,7 @@ def connect(ssh_cmd, rhostport, python, stderr, options):
             pycmd = "'%s' -c '%s'" % (python, pyscript)
         else:
             pycmd = ("P=python2; $P -V 2>/dev/null || P=python; "
-                     "\"$P\" -c '%s'") % pyscript
+                     "exec \"$P\" -c '%s'") % pyscript
         argv = (sshl + 
                 portl + 
                 ipv6flag +