lib: fix display cputime-warning and walltime-warning

lib: fix display cputime-warning and walltime-warning

Before patch:
```
# vtysh -c "c t" -c "service cputime-warning 1" -c "service walltime-warning 2"
# vtysh  -c "show run" | grep warn
service cputime-warning 1000
service walltime-warning 2000
```

After patch:
```
# vtysh -c "c t" -c "service cputime-warning 1" -c "service walltime-warning 2"
# vtysh  -c "show run" | grep warn
service cputime-warning 1
service walltime-warning 2
```

Signed-off-by: Dmitrii Turlupov <dturlupov@factor-ts.ru>