FIx the linebuffering for stdlib.call library

[pirat89]

Actual behavior
Currently we are not able to use linebuffering in stdlib.call as we are not able to handle linebuffer correctly now. Sometimes it happens that the buffered string is broken because it
ends in the middle of utf8 character. We have a temporary solution now. For the correct solution, we should use codecs.StreamReader.

Possible reproducer
It is possible it will not work for everyone and I am lazy to create short reproducer just now (sorry)

# LANG=cs_CZ.utf8
# leapp upgrade --debug

Err output

[root@localhost ~]# LEAPP_DEVEL_SKIP_RHSM=1 leapp upgrade
Process Process-61:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 59, in _do_run
    target_actor(logger=logger, messaging=messaging).run(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 299, in run
    self.process(*args)
  File "/usr/share/leapp-repository/repositories/system_upgrade/el7toel8/actors/rpmscanner/actor.py", line 26, in process
    ], split=True)['stdout']
  File "/usr/lib/python2.7/site-packages/leapp/libraries/stdlib/__init__.py", line 142, in run
    result = _call(args, callback_raw=callback_raw)
  File "/usr/lib/python2.7/site-packages/leapp/libraries/stdlib/call.py", line 156, in _call
    **extra
  File "/usr/lib/python2.7/site-packages/leapp/libraries/stdlib/call.py", line 46, in _multiplex
    linebufs[fd] += read.decode(encoding)
  File "/usr/lib64/python2.7/encodings/utf_8.py", line 16, in decode
    return codecs.utf_8_decode(input, errors, True)
UnicodeDecodeError: 'utf8' codec can't decode byte 0xc2 in position 79: unexpected end of data**Expected behavior**

[pirat89]

Fixed by PR #490