Simple refactor to stop passing around an unused parameter

Simple refactor to stop passing around an unused parameter

There is no functional change in this patch.  However, this refactor
allows another patch [1] to make a simpler change to the code base.

This patch includes localizing the initialization of "internal_cidrs"
to a later spot in the "process_router" method.  This has the
advantage that it will be initialized under the same condition that it
will be used.

[1] https://review.openstack.org/#/c/109776

Change-Id: I3ca993d0a33b7c925526099e97999c9d9f76efc2