Write unit tests for testing variable overriding

PIVOTAL_TRACKER_STORY_URL=https://www.pivotaltracker.com/story/show/82827566
PIVOTAL_TRACKER_STORY_TYPE=feature
PIVOTAL_TRACKER_STORY_ESTIMATE=5
PIVOTAL_TRACKER_STORY_OWNER=karldominguez
PIVOTAL_TRACKER_STORY_REQUESTER=katdecastro
PIVOTAL_TRACKER_STORY_LABELS=envetcd, soaf