txstatsd/graphite: switch back to package

require_package interacts badly (with require => Package[]?) and makes puppet
fail non-deterministically (!) on hosts, revert back to package. note this
means that txstatsd is missing python-carbon dependency when installed on
hosts that don't include graphite.

Change-Id: Ib66958e3b2f24b93d8779e9283e2daee435f74e7