Use envetcd to retrieve etcd config data instead of etcdctl in systemd unit file

PIVOTAL_TRACKER_STORY_URL=https://www.pivotaltracker.com/story/show/82827528
PIVOTAL_TRACKER_STORY_TYPE=feature
PIVOTAL_TRACKER_STORY_ESTIMATE=3
PIVOTAL_TRACKER_STORY_OWNER=carlsaturnino, kdominguez
PIVOTAL_TRACKER_STORY_REQUESTER=kdecastro
PIVOTAL_TRACKER_STORY_LABELS=envetcd, soaf