bgpd: Add link bandwidth route-map commands

bgpd: Add link bandwidth route-map commands

Implement route-map option to set the link-bandwidth extended
community. The command is of the form:

set extcommunity bandwidth <(1-26214400)|cumulative|num-multipaths>
[non-transitive]

The options available are to specify the actual bandwidth value in
Mbps, base it on the cumulative downstream bandwidth or base it on
the number of multipaths. The last option is based on
https://tools.ietf.org/html/draft-mohanty-bess-ebgp-dmz. Further,
in alignment with the use case described in this IETF draft, the
extended community is encoded as transitive by default. There is an
option available to specify that it should be non-transitive.

The link-bandwidth itself is carried in bytes per second as specifed in
https://tools.ietf.org/html/draft-ietf-idr-link-bandwidth

Note: This commit only handles the processing for bandwidth specifed
as a value; subsequent commits will handle the processing of the other
options.

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