Disable PUT for IPv6 subnet attributes

In Juno we are not ready for allowing the IPv6 attributes on a subnet
to be updated after the subnet is created, because:
- The implementation for supporting updates is incomplete.
- Perceived lack of usefulness, no good use cases known yet.
- Allowing updates causes more complexity in the code.
- Have not tested that radvd, dhcp, etc. behave OK after update.

Therefore, for now, we set 'allow_put' to False for the two IPv6
attributes, ipv6_ra_mode and ipv6_address_mode. This prevents the
modes from being updated via the PUT:subnets API.

Closes-bug: #1378952

Change-Id: Id6ce894d223c91421b62f82d266cfc15fa63ed0e