bigswitch: Use lazy logging interpolation

bigswitch: 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 bigswitch via a pylint
test.  Other occurrences are addressed elsewhere.

Change-Id: I23fd4e7d0e4173e753a124d62a1563ce6ac674ca
Partial-Bug: #1404788