Add --compat-disk-labels option

[deajan]

So I use smartmon.sh for years now.
Migrating to smartmon.py, I really got frustrated because metrics like:

smartmon_device_info{disk="/dev/sda" [...]}

became

smartmon_device_info{device="/dev/sda", disk="0" [...]}

So basically I have to batch update all my instances, or develop new dashboards (I can't get disk or device variable in grafana, one has to choose).

So my proposition is to add --compat-disk-labels to smartmon.py in order for the latter to generate metrics with the same disk name as previously found in smarmon.sh.

Modification is applied explicitly, so no behavior changes for existing setups.

[dswarbrick]

If you really need this backwards compatible label, you can achieve it with a Prometheus relabel rule.

[deajan]

The problem being that both metrics have a disk entry, and only the ones from smartmon.py would need relabeling.
I honestly think it's easier to install new setups so they can be compatible with existing ones (the less work).

[deajan]

Closed in favor of #256