psutils: Add --get-model option

Add a --get-model command line option to the psutils tool. This option
obtains the PSU model using information in sysfs.

Supports both methods of obtaining informat

psutils: Add --get-model option

Add a --get-model command line option to the psutils tool. This option
obtains the PSU model using information in sysfs.

Supports both methods of obtaining information about a PSU:
* psu.json file
* D-Bus

Tested:
* Verified --get-version still works
* With psu.json file
* Without psu.json file
* Verified new --get-model property works
* With psu.json file
* Without psu.json file
* Tested all error paths
* See the following gist for the complete test plan:
https://gist.github.com/smccarney/859ffaaa94ce12992af1b24e6c899962

Change-Id: If1be01f4b70ad9d80ce01402c57730990fd2c2ea
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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