This PS will check config file passed from command line is actually
existing or not in tempest run command. If file exists, only then it
will go for set config path otherwise will fail with value error.
Also adding and modifying unit test cases for this change.
Partially-Implements: blueprint tempest-cli-unit-test-coverage
Change-Id: I09d756be69cb3a9be8d0638c41d45a089d62b238
Closes-Bug: #1808473