bgpd: allow using rtt in route-map's set metric

bgpd: allow using rtt in route-map's set metric

Useful when the BGP neighbors are over tunnels that have large
differences in geographic distances and RTTs. Especially useful
for DMVPN setups to allow preferring closes hub.

The parameter is added as new alias command as otherwise it seems
the command parser is not able to match it properly (it seems
merging is done for the various 'set metric' route-map objects in
different routing engines). For same reason also they are listed
as three separate options: optional +/- seems not possibly easily.

Related research papers:
http://www.pps.univ-paris-diderot.fr/~jch/research/delay-based.pdf
http://arxiv.org/pdf/1309.0632.pdf

Paper on similar extension to Babel:
http://www.pps.univ-paris-diderot.fr/~jch/research/rapport-jonglez-2013.pdf

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit ef757700d0fd51dc0b46df9d3631208919f9b779)