Add CDH-from-packages option and flume agent

If whirr.cm.use.packages=true is specified, CDH packages will be installed (except for impala - that's still separate, but I'm holding off on that until it's got a permanent repo location on archive.cloudera.com rather than beta.cloudera.com) and parcels won't be.

Also added flume agent handler and related handwaving magic. =)