Revert shebang change for InstanceHA startup script

In Id915ded03ae5a471ffa2dca13e2da90021279f63 the shebang for
check-run-nova-compute has been changed to use /usr/bin/env, which
broke exec'ing the script:

  + exec /var/lib/nova/instanceha/check-run-nova-compute
  Running command: '/var/lib/nova/instanceha/check-run-nova-compute '
  /usr/bin/env: python -utt: No such file or directory

Revert the change for this script.

Change-Id: I96f82421638581122d687492f7420fd5724a7207
Closes-Bug: #1785245
(cherry picked from commit 9668e7b)

extraconfig/tasks/instanceha/check-run-nova-compute

@@ -1,4 +1,4 @@
-#!/usr/bin/env python -utt
+#!/bin/python -utt
 
 import os
 import sys