vtysh: fix duplicated output of key chain configuration

vtysh: fix duplicated output of key chain configuration

When both ripd and eigrpd run at the same time, all key configuration in
key chain node is duplicated. This change adds a concept of nested nodes
into vtysh to fix the issue.

Before:
```
key chain test
 key 1
  key-string 1
 exit
 key 1
  key-string 1
 exit
exit
!
```
After:
```
key chain test
 key 1
  key-string 1
 exit
exit
!
```

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>