Run rally ./exercise.sh inside post_test_hook

Since ./exercise.sh is run via sudo -H, environment variables
are not available for exercises/rally.sh. Some variables are
needed to be preserved, so we run ./exercise.sh in post_test_hook
without sudo.

Change-Id: I4c147219037d9358e8abda32f2dca7bde5706f83
Merge "Run rally ./exercise.sh inside post_test_hook"