[ssdhealth] Check for default device before falling back to discovery

[vivekrnv]

What I did

Some platforms may have multiple primary disks which makes it ambiguous to determine the device when there is no device specific in the ssdhealth command

How I did it

Update the show platform command to check platform.json

How to verify it

1. UT's

vkarri@85964d14e169:/sonic/src/sonic-utilities$ pytest-3 tests/show_platform_test.py -k "ssdhealth" -v
collected 8 items / 6 deselected / 2 selected

tests/show_platform_test.py::TestShowPlatformSsdhealth::test_ssdhealth PASSED                                                                                                                                [ 50%]
tests/show_platform_test.py::TestShowPlatformSsdhealth::test_ssdhealth_default_device PASSED                                                                                                                 [100%]

2. Verified the CLI is printing the health output for expected device

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

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