BGP: Enable multiple instance support by default

BGP: Enable multiple instance support by default

Quagga BGP needed a config 'bgp multiple-instance' in order to be able
to configure and use VRFs. Since this support is intrinsic to the
implementation, make this configuration on by default. Corresponding
change to 'show running-config' (and write) to display only if "no" is
configured.

This change will eliminate one unnecessary step in the configuration.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-10070
Reviewed By: CCR-4383
Testing Done:  Manual