Fixed wrong config parameter

Fixed wrong config parameter

Job is failing because it uses REDIS_HOSTNAME config parameter instead
of REDIS_HOST. As REDIS_HOSTNAME does not exist job crashes this way:
Traceback (most recent call last):
  File "jenkins-scripts/job_load_repos.yaml.py", line 74, in <module>
    sys.exit(main())
  File "jenkins-scripts/job_load_repos.yaml.py", line 71, in main
    return load_repos(os.path.join(prjdir, 'repos.yaml'))
  File "jenkins-scripts/job_load_repos.yaml.py", line 26, in load_repos
    int(os.getenv('REDIS_PORT')))
  File "/srv/jenkins/jobs/load-repo-conf/workspace/jenkins-scripts/common/backenddb.py", line 141, in __init__
    self._redis.exists('anything')
  File "/usr/lib/python2.7/site-packages/redis/client.py", line 465, in exists
    return self.execute_command('EXISTS', name)
  File "/usr/lib/python2.7/site-packages/redis/client.py", line 334, in execute_command
    connection.send_command(*args)
  File "/usr/lib/python2.7/site-packages/redis/connection.py", line 287, in send_command
    self.send_packed_command(self.pack_command(*args))
  File "/usr/lib/python2.7/site-packages/redis/connection.py", line 269, in send_packed_command
    self.connect()
  File "/usr/lib/python2.7/site-packages/redis/connection.py", line 214, in connect
    sock = self._connect()
  File "/usr/lib/python2.7/site-packages/redis/connection.py", line 226, in _connect
    sock.connect((self.host, self.port))
  File "/usr/lib64/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
TypeError: coercing to Unicode: need string or buffer, NoneType found

Fixes: #1355

Change-Id: I37a60fc7b960301fe06b1cc1efc45189c471c532
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>