bgpd: store the bgp as identifier in the configured as-notation

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>