bundle: only encode shlex.split arg on Python 2.x
bundle: only encode shlex.split arg on Python 2.x

Before, we would encode all str objects to bytes objects, and this
causes an error in Python 3:

  >>> shlex.split(b'foo bar baz')
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/lib/python3.3/shlex.py", line 276, in split
      return list(lex)
    File "/usr/lib/python3.3/shlex.py", line 266, in __next__
      token = self.get_token()
    File "/usr/lib/python3.3/shlex.py", line 93, in get_token
      raw = self.read_token()
    File "/usr/lib/python3.3/shlex.py", line 121, in read_token
      nextchar = self.instream.read(1)
  AttributeError: 'bytes' object has no attribute 'read'

zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli,zerovm/zpm,zerovm/zpm,zerovm/zpm,zerovm/zpm,zerovm/zerovm-cli,zerovm/zerovm-cli