Clean up sysctl parameters.

This patch reorganizes the sysctlfile module and sysctl resource into a
reworked sysctl module and configures Puppet to manage /etc/sysctl.d
recursively. This means Puppet _will_ purge defaults installed into
/etc/sysctl.d by default. However, the defaults installed by Ubuntu Precise
have been expressed as Sysctl::Parameters['ubuntu precise defaults'] in class
base::sysctl.

Change-Id: Ib294b691dad8500c2e0cd39896882f8cf4f3a286