ansible: Add cleanup for grafana cache under host machine

ansible: Add cleanup for grafana cache under host machine

Previously cleaning cache directory for grafana(/var/lib/grafana)
was missing and, because of that, changing influxdb_host variable
value between one ansible deployment and another was not working.

It was a problem because scripts in grafana container are expecting
that cache directory will be empty during first run and, if some
dashboards exists in cache already (even if they are left by
previous deployment), old dashboards will be left untouched with
possibly wrong configuration. This bug happened during deploying
3 nodes scenarios more than once (collectd, grafana and influlxdb
hosted on separate machines).

Now cleanup is done by default and can be skipped using
'clean_grafana_cache=false' value.

Change-Id: Ib67cc3437b8a3a2c2930dd55cba44aef8e352cf2
Signed-off-by: Radoslaw Jablonski <radoslawx.jablonski@intel.com>