bgpd: prompt should be given only for the real change

bgpd: prompt should be given only for the real change

Prompt nothing for an empty (and failed) operation.  Take
`bgp graceful-restart` as an example:

Before:
```
anlan(config-router)# bgp graceful-restart
Graceful restart configuration changed, reset all peers to take effect
anlan(config-router)# bgp graceful-restart
Graceful restart configuration changed, reset all peers to take effect
anlan(config-router)#
```

After:
```
anlan(config-router)# bgp graceful-restart
Graceful restart configuration changed, reset all peers to take effect
anlan(config-router)# bgp graceful-restart
anlan(config-router)#
```

Signed-off-by: anlan_cs <anlan_cs@tom.com>