ovs.daemon: Fix semantics of --pidfile option.

ovs.daemon: Fix semantics of --pidfile option.

The --pidfile option is supposed to work like this:

   * Without --pidfile, you don't get a pidfile.
   * With --pidfile, you get the default pidfile.
   * With --pidfile=FILE, you get FILE as your pidfile.

However, it actually worked like this:

   * Without --pidfile, you got the default pidfile.
   * With --pidfile, you got no pidfile at all.
   * With --pidfile=FILE, you got FILE as your pidfile.

This is because of the semantics of "default" in argparse.  It is
documented as:

    The default keyword argument of add_argument(), whose value defaults to
    None, specifies what value should be used if the command-line argument
    is not present.  For optional arguments, the default value is used when
    the option string was not present at the command line.

We actually want "const", which is documented under the description of
nargs="?" as:

    If no command-line argument is present, the value from default will be
    produced.  Note that for optional arguments, there is an additional
    case - the option string is present but not followed by a command-line
    argument.  In this case the value from const will be produced.

Bug #7533.