Redfish: implement obtaining PowerRestorePolicy (#9824) (#9824)
This property ("The desired power state of the system when power is
restored after a power loss.") was added in ComputerSystem.v1_6_0 which
became part of 2018.3 Redfish release.
Example result from querying OpenBMC's bmcweb Redfish server:
```
entries:
- - system_uri: /redfish/v1/Systems/system
- AlwaysOff
ret: true
```
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
(cherry picked from commit 129f51cf9d)
Co-authored-by: Paul Fertser <fercerpav@gmail.com>