LV performance monitoring

[lejmr]

This pull request adds autodiscovery for linear LVs and captures its performance. Subsequently, graphs are prepared for each LV

[hairmare]

@paraenggu PTAL

[paraenggu]

I'm really sorry @lejmr I completely forgot about this PR :-(

Thanks a lot for your contribution. Having access to I/O performance metrics could be really helpful.

However, I think this shouldn't be added to this LVM template, but rather put into a separate generic Linux block stats template. The user parameters can be used for other block devices as well and aren't limited to LVM devices.

The low-level discovery of the block devices could be based on /proc/diskstats or sub-entries of the /sys/class/block/ directory for Zabbix installations which don't have support for built-in low-level discovery of block devices (< 4.4.0).