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
1551b2b3d6