Add Link header handling to PowerSubsystem

This commit is to update PowerSubsystem to respond to HEAD requests
in the way the redfish spec requires.

Tested:
HEAD /redfish/v1/Chassis/<str>/PowerSubsystem returns a correct
looking Link header.
Following that Link header resolves the schema file.

Get /redfish/v1/Chassis/BadChassis/PowerSubsystem returns 404.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I2f2a0e80b74471ddc037e899cfaf3ba307741475