ovs_dvr: Use lazy logging interpolation

ovs_dvr: 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 ovs agent via a pylint
test.  Other occurrences are addressed elsewhere.

Change-Id: Ie215f33818cc5c16e570ace4c750df5371d27e09
Partial-Bug: #1404788