common.unittest.tools: sync slf4j-api version

common.unittest.tools: sync slf4j-api version

The slf4j-api version used in common.unittest.tools is out of sync with
both core controller and within lispflowmappng project.  Define the
slf4j.version property in lispflowmapping-commons as 1.7.2 and then
update common.unittest.tools dependency to use slf4j.version property
instead of hardcoded 1.6.1.

I was unable to successfully test this change due to other unrelated
test failures.

Change-Id: I693b9e63b9a9cedf446ae87fcf2a73e81aac89d3
Signed-off-by: Chris Wright <chrisw@sous-sol.org>