bgpd: Additional options for generating link bandwidth

bgpd: Additional options for generating link bandwidth

Implement the code to handle the other route-map options to generate
the link bandwidth, namely, to use the cumulative bandwidth or to
base this on the number of multipaths. In the latter case, a reference
bandwidth is internally chosen - the implementation uses a value of
1 Gbps.

These additional options mean that the prefix may need to be advertised
if there is a link bandwidth change, which is a new criteria. Define a
new path (change) flag to support this and implement the advertisement.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>