Fix association creation

This change contains two fixes:

1) phosphor-health-monitor will now not exit with an
xyz.openbmc_project.Common.Error.ResourceNotFound error even when the
inventory it's looking for (/xyz/openbmc_project/inventory/system) is
not found. The sensor objects will still be created; they will just not
have associations with the system.

2) phosphor-health-monitor now listens for the "BMC inventory activated"
InventoryAdded singal. Upon detecting this signal, it will recreate the
health sensors with associations.

Tested by stopping and restarting
xyz.openbmc_project.Inventory.Manager.service, saw the health sensors
disappear and re-appear from the curl command against the
https://${bmc}/redfish/v1/Chassis/bmc/Sensors RedFish resource.

Signed-off-by: Sui Chen <suichen@google.com>
Change-Id: I37abe0b680038f8989181ac816ac67bfa3ee60bf
2 files changed