Adding Swift proxy-server memcache metadata

Swift proxy-servers use memcache to store and share metadata. This
change adds swift.proxy-memcache metadata to the swift-source and
swift-storage-source yaml templates modelled on the existing
swift.devices metadata.  This metadata will be consumed by the
swift-proxy/os-config-applier/etc/swift/proxy-server.conf element
if the metadata exists.

Change-Id: If0b5724f69e7ec1c98e4dbdbeb9f08c4a18151b6
Merge "Adding Swift proxy-server memcache metadata"