Add multi-host support for PostCodes LogService

Add support for multi-host for all GET and POST method requests under

/redfish/v1/Systems/{computerSystemId}/LogServices/PostCodes/

redfish resource.

Tested:
Redfish-service-validation passes on single-host machine as well as on
yv4 in qemu.

Every postcode entry is displayed in web-ui on single-host and shown
in the curl output when requested on yv4 qemu.

POST has been tested on single-host hardware inside web-ui and yv4 qemu
machine with curl. (Postcodes have been copied over to the qemu machine
manually from a single-host machine). No regressions observed.
The postcode dir for the specific host is cleared successfully
on both single- and multi-host machine after the POST.

Change-Id: Ie04cb160a1f2756a04be68e6675a6cecc5f09117
Signed-off-by: Oliver Brewka <oliver.brewka@9elements.com>

Author: mox669