remove/replace memcached servers via etcd

This makes it possible for core-cache to automatically replace and
remove memcached servers when the etcd entries change.

Right now, changing an existing memcached server with the same
memcached server is considered a no-op..
make the etcd prefix configurable

The etcd prefix for the keys used to set up the memcached servers
to be used by core-cache is now configurable.

"/service/memcached" is still the default.
make the path to the etcd cluster configurable

This adds another flag which allows core-cache to work with etcd
clusters without any local nodes.
Merge pull request #6 from unclejack/new-features-and-improvements

feat(core-cache): configurable etcd prefix & etcd cluster path, allow removal of memcache instances