bgpd: store the bgp as identifier in the configured as-notation
This is a preliminary work to handle various ways to configure
a BGP Autonomous System. When creating a BGP instance, the
user may want to define the AS number as a dotted value,
instead of using an integer value.
To handle both cases, an as_pretty char attribute will store
the as number as it has been given to the vtysh command:
router bgp <as number>
Whenever the as integer of the BGP instance was dumped,
the as_pretty original format is used.
The json output reuses the integer value to keep backward
compatibility with old displays.
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>