adding the command "ln" to the CommandFilter.

I was getting the error: "/usr/bin/nova-rootwrap: Unauthorized command:
ln -sf...Filter not found", while running the "nova-scheduler". Due to
this, there was an error on the horizon dashboard "Host not found". I
fixed it by adding the "/usr/bin/ln" command to the CommandFilter file.

Signed-off-by: x1b2j <gx1b2j@gmail.com>