ceph-dev-new-build/build/setup_rpm: install python2 for el7

ceph-dev-new-build/build/setup_rpm: install python2 for el7

otherwise we will have

Traceback (most recent call last):
  File "/tmp/venv.pAISkJ09vG/bin/chacractl", line 6, in <module>
    main.ChacraCtl()
  File "/tmp/venv.pAISkJ09vG/lib/python3.6/site-packages/chacractl/main.py", line 38, in __init__
    self.main(argv)
  File "/tmp/venv.pAISkJ09vG/lib/python3.6/site-packages/chacractl/decorators.py", line 68, in newfunc
    return f(*a, **kw)
  File "/tmp/venv.pAISkJ09vG/lib/python3.6/site-packages/chacractl/main.py", line 82, in main
    parser.dispatch()
  File "/tmp/venv.pAISkJ09vG/lib/python3.6/site-packages/tambo/dispatcher.py", line 21, in dispatch
    result = instance.main()
  File "/tmp/venv.pAISkJ09vG/lib/python3.6/site-packages/chacractl/api/exists.py", line 62, in main
    return self.head(url)
  File "/tmp/venv.pAISkJ09vG/lib/python3.6/site-packages/chacractl/decorators.py", line 68, in newfunc
    return f(*a, **kw)
  File "/tmp/venv.pAISkJ09vG/lib/python3.6/site-packages/chacractl/util.py", line 65, in inner_wrapper
    for counter in xrange(times):
NameError: name 'xrange' is not defined

Signed-off-by: Kefu Chai <kchai@redhat.com>