tools: fix frr traceback on bgp neighbor bfd deletion

tools: fix frr traceback on bgp neighbor bfd deletion

Problem reported that with certain configs, when the user
deleted a "neighbor x.x.x.x bfd 4 100 100" statement from
frr.conf and then reloaded, a traceback was seen and the
deletion did not succeed.  Found that in some scenarios
it was possible to have something in lines_to_add that
was in a different context and when the re.search was
attempted, it found an empy line and was unhappy.  This
fix avoids trying to search in the wrong context.

Ticket: CM-29145
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>