Fix manila-tempest-job to use exported env var

Manila's tempest job uses two hooks - pre and post, in 'pre' hook is defined
environment variable, that is expected to be used in 'post' function.
But it wasn't exported and post hook takes empty value.

Partially implements bp: tempest-job

Change-Id: I730d9e004a0ffe041e1f3ac4bb9566217556a07b
Merge "Fix manila-tempest-job to use exported env var"