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
Added CHANGELOG for 0.20.0.
Fixed style issues in CHANGELOG.
Updated configure.ac to 0.21.0.
Added HTTP endpoint changes and deprecations to CHANGELOG for 0.20.0.
Modify execute cli to run docker image