Add psu hardware revision to psushow table

[alexrallen]

What I did

Added a field for hardware revision to the psushow output table.

How I did it

Defined new psu status field sourced from "revision" field in STATE_DB added in alexrallen/sonic-platform-daemons#1 and adds it to the table output with the header "HW Rev"

All relevant unit tests were also updated.

How to verify it

Execute show platform psustatus and verify that "HW Rev" column is present and appropriately populated depending on the platform.

Previous command output (if the output of a command-line utility has changed)

PSU    Model    Serial      Voltage (V)    Current (A)    Power (W)  Status    LED
-----  -------  --------  -------------  -------------  -----------  --------  -----
PSU 1  N/A      N/A               12.08           3.62        43.75  OK        green

New command output (if the output of a command-line utility has changed)

PSU    Model          Serial        HW Rev      Voltage (V)    Current (A)    Power (W)  Status    LED
-----  -------------  ------------  --------  -------------  -------------  -----------  --------  -----
PSU 1  MTEF-PSF-AC-A  MT1621X15246  A3                11.97           4.56        54.56  OK        green