Convert severity fields in Thresholds into strings

[RashmicaG]

At the moment severity fields are ints 0-4 which correspond one to one with a threshold type:

Threshold	Severity
Warning	0
Critical	1
PerformanceLoss	2
SoftShutdown	3
HardShutdown	4

Change these to a string enum so it's clear which threshold is being used in config files. This will require updating the schema here, dbus-sensors and phosphor-virtual-sensors.