bgpd: Exchange hostname capability and display hostnames in output

bgpd: Exchange hostname capability and display hostnames in output

This patch adds a hostname capability. The node's hostname and
domainname are exchanged in the new capability and used in show command
outputs based on a knob enabled by the user. The hostname and domainname
can be a maximum of 64 chars long, each.

Signed-off-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Vivek Venkataraman <vivek@cumulusnetworks.com>
Ticket: CM-5660
Reviewed By: CCR-2563
Testing Done: