Don't allow update of ipv6-ra-mode and ipv6-address-mode

These two parameters are not updatable from the nuetron server.
And therefore for subnet-update command, they should not be displayed
in the help message.

Change-Id: I3a6abc3a4e3030d73715d55c340a77b44d41f140
Closes-Bug: #1373417
Merge "Don't allow update of ipv6-ra-mode and ipv6-address-mode"