Add swift.proxy-memcache metadata to proxy-server.conf

We're building a list of proxies to add to the memcache stanza
in /etc/swift/proxy-server.conf. The list is built as metadata
in the swift-source and swift-storage-source heat yaml templates.
This element change will consume the metadata if it exists.

Change-Id: I0e6b68086246dd731ac2e9a868e63eedb3ac3d6d
Merge "Add swift.proxy-memcache metadata to proxy-server.conf"