cassandra: add sysctl.d tunable

The Cassandra packages ship a sysctl file setting vm.max_map_count. We
do recurse => true, purge => true for /etc/sysctl.d so we clean these
up. Reinstate the setting by explicitly using sysctl::parameters for
this.

(this is currently being done via a very hackish manner out of site.pp;
this is temporary until Cassandra gets a puppet class)

Change-Id: I96d1c6d53dd8fc3fe9f493233d5c026ef146e4d1
Merge "cassandra: add sysctl.d tunable" into production