Sysstat won't enable unless the ENABLED switch has been set to true.
Passing openstack_host_sysstat_enabled as true inside jinja will end
up as a "True" value inside the /etc/default/sysstat rather then the
expected "true" value and sysstat won't start.
The added jinja filter lower will make sure that the boolean is always
stored in lower case.
Additionally openstack_host_systat_enabled was renamed to
openstack_host_sysstat_enabled to better reflect the dependency to sysstat
Change-Id: I6361433f990f96ca37dac7055062c17c2940bf70
Closes-Bug: #1530979
ee8f634517