gluon-l3roamd: site: make node_client_prefix6 optional+deprecated

The site.node_client_prefix6() is only used internally by the l3roamd
protocol. Therefore it is unnecessary to expose it to an administrator.

Instead, if node_client_prefix6 is unspecified in the site, generate an
IPv6 Unique Local Address prefix from the site domain_seed.

This updates the site documentation as well and marks this setting as
both optional and deprecated.

Note: If you had the node_client_prefix6 specified before and want to
use the new autogeneration from the domain_seed instead then this will
break compatibility and will need a gluon-scheduled-domain switch.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>