Allow substitution of values in executed scripts

The executed scripts from the monitors should allow calling executables without having to create a wrapper script for them. This can be done by substituting keywords with values provided by MaxScale.

$INITIATOR should be replaced with the IP and port of the server which initiated the event.
$EVENT should be replaced with the type of the event.
$NODELIST should be replaced with the list of IPs and ports of the running servers.