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
could you add this file to be copied to the correct location in the vagrant bootstrap.sh?  Currently the service will fail to start with these changes because the raingauge_rc file is modified but this script isn't there.