Adds support for custom pt-stalk triggers

This allows us to provide multiple test cases in which pt-stalk should collect server data
To use original pt-stalk functionality, leave PT_STALK_FUNCTION in scripts/raingauge_rc empty
Add/remove custom triggers in raingauge_triggers.sh
Refer to /var/log/pt-stalk.log to see which trigger caused collection
Adds function get_delta in raingauge_triggers.sh to calculate change in counter values