Remove "optional" memcached interface logic

Filed after conversation in http://review.couchbase.org/c/kv_engine/+/141254

Background:

memcached added support for binding to IPv4 and IPv6 before we started the company and would "by default" bind to all both IPv4 and IPv6 if it could find it. As part of adding support for IPv6 in our product the parameters was added; initially as a boolean variable. The problem was that ns_server would set it to true even if there wasn't an IPv6 stack on the machine; causing memcached to refuse to start as it couldn't bind to the IPv6 address. ns_server would then use "optional" as a way to prevent memcached from refusing to start when there was no IPv6 on the host.

Proposal:

ns-server should discover what interfaces the host/operating system provides provides (dualstack, only one of them etc) and it'll just work out of the box for customers. 

We can then get rid of "optional" logic in memcached interfaces.