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

elements/swift-proxy/README.md

@@ -8,3 +8,5 @@ swift:
 keystone:
     host: 127.0.0.1
       - The IP of the keystone host to authenticate against
+proxy-memcache:
+    Comma-separated list of proxy servers in memcache ring

elements/swift-proxy/os-apply-config/etc/swift/proxy-server.conf

@@ -20,6 +20,9 @@ use = egg:swift#healthcheck
 
 [filter:cache]
 use = egg:swift#memcache
+{{#swift.proxy-memcache}}
+memcache_servers = {{swift.proxy-memcache}}
+{{/swift.proxy-memcache}}
 
 [filter:slo]
 use = egg:swift#slo