Add optional hysteresis field to Thresholds

This allows applications to use a hysteresis when clearing threshold
alerts. For example, if a critical high temperature sensor threshold
value is set to 50 with a hysteresis of 5, then the alarm property in
the threshold interface is set when the temperature crosses 50 when
increasing, and when the temperature decreases it isn't cleared until
it gets to 45.

At this point, this is only needed by the phosphor-virtual-sensor code
which is going to get its sensor config from entity manager

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Change-Id: I3d00746343c8a8c672aa77a702bcf65705d20b4c
diff --git a/schemas/legacy.json b/schemas/legacy.json
index cb2a978..47a6c7b 100644
--- a/schemas/legacy.json
+++ b/schemas/legacy.json
@@ -682,6 +682,9 @@
                         "Direction": {
                             "type": "string"
                         },
+                        "Hysteresis": {
+                            "type": "number"
+                        },
                         "Label": {
                             "type": "string"
                         },