zebra: fix mpls command

zebra: fix mpls command

Configured with "mpls label bind 1.1.1.1/32 explicit-null", the running
configuration is:
```
!
mpls label bind 1.1.1.1/32 IPv4 Explicit Null
!
```

After this commit, the running configuration is:
```
!
mpls label bind 1.1.1.1/32 explicit-null
!
```

And add the support for the "no" form:
```
anlan(config)# mpls label bind 1.1.1.1/32 explicit-null
anlan(config)# no mpls label bind 1.1.1.1/32 explicit-null
```

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