mgr/zabbix: monitoring template improvements

mgr/zabbix: monitoring template improvements

The changes need some review because i am currently not able to test this in a environment.
- added a host screen which displays all graphs in a convenient way
- replaced AVG by MAX Function in Graphs where maximum values are of
  high interest from operational view
- changed graph type to stacked where combined behavior is of interest
  (i.e. bandwidth and storage consumption)

Signed-off-by: Marc Schoechlin <ms@256bit.org>