Fixed MESOS-935: Group should tell MasterDetector "no memberships
detected" when it locally times out.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/17156
Improved Group to take label as an option.

Review: https://reviews.apache.org/r/17171
Updated detector to understand old (unlabelled) and new (labelled)
master znode formats.

Review: https://reviews.apache.org/r/17172
Updated master and contender to write znode(s) in the new (labelled)
format.

Review: https://reviews.apache.org/r/17173
Updated upgrades.md for 0.15.0 -> 0.16.0.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/17216