Set path to config file in tempest

Fix critical bug in preparation for Tempest config generation:
when `TEMPEST_CONFIG_FILE` is not presented, variables `TEMPEST_CONFIG` and
`OS_TEST_PATH` are set wrong.

Change-Id: Ic3011e7268f9d7d989a29295a126b247ddc6df2c
Merge "Set path to config file in tempest"