Improve sysinv handling of host UUID in /etc/platform/platform.conf file

[Bart Wensley]

Brief Description
-----------------
There are a couple potential issues with the handling of the host UUID, which is written into the /etc/platform/platform.conf file on each host (as the UUID entry):
1. The sysinv-agent will detect a missing UUID in this file and write it to the file (based on what is in the sysinv DB). However, if the UUID in the file is present, but not correct, it will be left as it is.
2. The sysinv-fpga-agent is using the UUID from the /etc/platform/platform.conf file to control its startup and to report inventory. The sysinv-agent uses the host UUID from the sysinv DB for the same purpose. We need to evaluate whether the sysinv-fpga-agent should be changed to behave like the sysinv-agent.

Severity
--------
Minor: The above issues caused firmware update related failures after an AIO-SX upgrade. However, the fix for bug 1891766 removed the trigger that exposed the above two issues.

Steps to Reproduce
------------------
The issues happened in an AIO-SX upgrade.

Expected Behavior
------------------
See above

Actual Behavior
----------------
See above

Reproducibility
---------------
Reproducible before bug 1891766 was fixed

System Configuration
--------------------
AIO-SX (One node system)

Branch/Pull Time/Commit
-----------------------
stx.4.0

Last Pass
---------
Never

Timestamp/Logs
--------------
N/A

Test Activity
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Feature Testing

Workaround
----------
None

[Ghada Khalil (gkhalil)]

Low priority - code cleanup. Current code will not cause issues as https://bugs.launchpad.net/starlingx/+bug/1891766 is already fixed.

[John Kung (john-kung)]

This is being addressed by Task: https://storyboard.openstack.org/#!/task/45628 to merge fgpa agent into sysinv agent. In such case, the host_uuid will be consistent based on the sysinv db view updated by sysinv-agent.

[Ramaswamy Subramanian (rsubrama)]

https://review.opendev.org/c/starlingx/tools/+/845824