MySQL find and replace is optional now

It makes sense to run mysql:find-replace in many cases.  However,
sometimes, the patterns are not supplied.  It makes sense to
check if they are supplied or not, and if not, then just skip
the running of the task with the debug message, rather than a
full blown exception.
Merge pull request #21 from QoboLtd/optional-mysql-find-replace

MySQL find and replace is optional now