envetcd should provide a few more variables by default

envetcd should expose the &#x60;--hostname&#x60;, &#x60;--system&#x60; and &#x60;--service&#x60; variables in its output as &#x60;ENVETCD_HOSTNAME&#x60;, &#x60;ENVETCD_IMAGE&#x60; and &#x60;ENVETCD_SERVICE&#x60; (if they are set) even if they do not exist in &#x60;etcd&#x60;

PIVOTAL_TRACKER_STORY_URL=https://www.pivotaltracker.com/story/show/84605868
PIVOTAL_TRACKER_STORY_TYPE=feature
PIVOTAL_TRACKER_STORY_ESTIMATE=1
PIVOTAL_TRACKER_STORY_OWNER=lukekingland
PIVOTAL_TRACKER_STORY_REQUESTER=jrubin
PIVOTAL_TRACKER_STORY_LABELS=envetcd, soaf