Create new IPv6 attributes for Subnets by client

This is the client implementation of bp ipv6-two-attributes.
Add two optional arguments:
 --ipv6-ra-mode
 --ipv6-address-mode

Partially implements bp ipv6-two-attributes

Change-Id: Ia922a9b6bb4e1dbd372174d48f29f5411c7b5674
Merge "Create new IPv6 attributes for Subnets by client"