Currently, swift-proxy config uses hosts in the swift-proxy-server group
to generate the list of memcached servers. However, memcached is
deployed to hosts in the memcached group.
This change fixes the memcached_servers option for swift-proxy to be the
same as other services.
Change-Id: Ib850a1bb2a504ac3e1396846ca3f1d9a30e8fca0
Closes-Bug: #1774313