BGP: Handle router-id correctly in config and display.

BGP: Handle router-id correctly in config and display.

BGP is currently displaying the in-use router-id in the config. This is
conditional on a CONFIG flag, however, that flag is set even when there
is no configured router-id and the router-id learnt from Zebra is in-use.
The CONFIG flag for router-id is redundant since there is a separate
variable for the configured value, so use that and deprecate the CONFIG
flag. This also makes BGP behave like OSPF.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>
Ticket: CM-8077, CM-8220
Reviewed By: CCR-3793
Testing Done: Manual verification (in 2.5-br)

Note: Imported from 2.5-br patch bgpd-fix-router-id-config-display.patch