Fixed update of env var in manila's job

It was running only first part of tests, defined in 'export' string, and never
second part, that is in condition of pre_test_hook.
Moved update of env var 'MANILA_TESTS' from pre_test_hook to top, because it
hasn't changed exported value.
Also fixed condition itself, that always was 'true'.

Change-Id: I292c45caaf66be0c7d0018ebc84686324298c2f1
Merge "Fixed update of env var in manila's job"