linuxbridge: Use lazy logging interpolation

linuxbridge: Use lazy logging interpolation

There are a small number of examples of "eager" interpolation in
neutron:
  logging.debug("foo %s" % arg)

These should be converted to perform the interpolation lazily within
the logging function, since if the severity is below the logging level
then the interpolation can be skipped entirely.

This change addresses all such examples found in linuxbridge agent via a
pylint test.  Other occurrences are addressed elsewhere.

Change-Id: I5cb3bbb401f5049760dfe5263a52bd43f23dcfd4
Partial-Bug: #1404788