Change a debug line to prevent UnicodeDecodeError issue

This debug line is causing tracebacks in the n-cpu logs for
tempest runs. Its because the logged data is sometimes unicode:

Traceback (most recent call last):
  File "/usr/lib/python2.7/logging/__init__.py", line 846, in emit
    msg = self.format(record)
  File "/opt/stack/new/nova/nova/openstack/common/log.py", line 710, in format
    return logging.StreamHandler.format(self, record)
  File "/usr/lib/python2.7/logging/__init__.py", line 723, in format
    return fmt.format(record)
  File "/opt/stack/new/nova/nova/openstack/common/log.py", line 674, in format
    return logging.Formatter.format(self, record)
  File "/usr/lib/python2.7/logging/__init__.py", line 467, in format
    s = self._fmt % record.__dict__
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 1234: ordinal not in range(128)
Logged from file http.py, line 153

The change used correct encoding error handling policy for the log
which may includes non-ascii char.

Closes-bug: 1320655
Change-Id: I97f5f14b9beddcceb7fbd371062caf5a38a62a20