Add unit tests for benchmark validation

Added following tests for benchmark validation

test_image_invalid_on_flavor
test_image_invalid_on_size
test_image_valid_on_missing_flavor_disk
test_requires_permission
test_tempest_test_no_config_install
test_tempest_set_exists_fail
test_tempest_set_exists
test_required_parameters

Updated return value for mocks in decorator

Change-Id: If9872664aed7a4c77d7dbc3f51f50f739c2e210a
Implements: blueprint improve-unit-test-coverage-rally