Fixed shebang and permissions for scripts.

* While packaging in RDO, rpmlints are failing by giving
  non-executable script errors, we need to remove the shebang
  from rally/plugins/workload/siege.py as it lies with in the
  rally package.
* unpack_plugins_samples.sh, instance_linpack.sh, instance_test.sh
  scripts does have executable permissions. we need to set the same
  for the same to avoid rpmlint errors[1.].

[1.] https://review.rdoproject.org/r/#/c/7007/5/openstack-rally.spec@105

Change-Id: Ibcab4b45a8d9dc06bf5bfbe0d8262f571da87eee

rally/plugins/workload/siege.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python
-#
 # All Rights Reserved.
 #
 #    Licensed under the Apache License, Version 2.0 (the "License"); you may