Support for RapidCommit option when doing DHCPv6 via networkd

When using networkd as the renderer, there is currently no way to configure networkd's RapidCommit option when using networkd as a DHCPv6 client.

Adding support for this is useful, as not all ISPs will issue a lease with RapidCommit=true, which is the current networkd default. This means that control over this setting is a requirement for netplan to be usable in these environments.

The resulting working DHCPv6 configuration in networkd's configuration files looks like this -

[Match]
Name=eth1

[Network]
DHCP=true
IPv6AcceptRA=true

[DHCPv6]
RapidCommit=false