bgpd: modify bgp as number output

bgpd: modify bgp as number output

A json AS number API is created in order to output a
given AS number. In order to keep backward compatibility,
if the as-notation uses a number, then the json is encoded
as an integer, otherwise the encoding will be a string.

For what is not relevant to running-configuration, the
as-notation mode is the one used for the BGP instance.

Also, the vty completion gets the configured 'as_pretty'
string value, when an user wants to get the available
BGP instances.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>