Disabled glog signal handler for mesos tests.

Review: https://reviews.apache.org/r/24780
Updated the upgrades.md for 0.20.0.

Review: https://reviews.apache.org/r/24781
Added documentation for Docker containerizer.

Review: https://reviews.apache.org/r/24775
Disabled glog signal handler for stout tests.

Review: https://reviews.apache.org/r/24783
Disabled glog signal handler for libprocess tests.

Review: https://reviews.apache.org/r/24784