[Reviewer: Graeme] Move memcached monit configuration out of /etc

Graeme,

Please can you review my fix to https://github.com/Metaswitch/sprout/issues/389?  The issue was that we were including memcached's monit configuration below /etc in the Debian package, but had modified it.  The fix is to include it in the package below /usr and then move it into the correct location as part of postinst.  I've live-tested.

Matt