enable_ceilometer support in proxy-server.conf.erb

If swift::enable_ceilometer set to true then enable ceilometer
middleware and add it to the swift proxy pipeline.

Change-Id: I753e530d1d6fd209131b7cf6f550e012bc6dc735
enable_ceilometer support for proxy.pp

Introduces a new variable, swift::enable_ceilometer. If true then
ensure the ceilometer-common package is installed before swift-proxy.

Change-Id: I96164521edeb34454e6d081ec589d5b59204d213