healthMonitor: call configured unit when threshold exceeded

Add threshold target implementation.
Call the system target unit configured in config file when threshold
value exceeded.

Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Change-Id: I31f0cf4df0c913f10c47ecb9fa24b59b7b9de0e5
diff --git a/healthMonitor.hpp b/healthMonitor.hpp
index d327cf0..b5013c8 100644
--- a/healthMonitor.hpp
+++ b/healthMonitor.hpp
@@ -125,6 +125,8 @@
     sdeventplus::utility::Timer<sdeventplus::ClockId::Monotonic> readTimer;
     /** @brief Read sensor at regular intrval */
     void readHealthSensor();
+    /** @brief Start configured threshold systemd unit */
+    void startUnit(const std::string& sysdUnit);
 };
 
 class HealthMon