CLUSTER-57. Use docker tags to allow platform version specification

issue: https://jira.kiji.org/browse/CLUSTER-57
review: https://review.kiji.org/r/2170
CLUSTER-63. Make bento-cluster installable through pip alone.

issue: https://jira.kiji.org/browse/CLUSTER-63
review: https://review.kiji.org/r/2195