CHANGELOG and docs/upgrades.md for 0.21.0.

Review: https://reviews.apache.org/r/27633
Update configure.ac version to 0.22.0
Fetcher uses Hadoop to fetch URIs with unknown schemes.

Review: https://reviews.apache.org/r/27483
Updated docs/configuration.md.

- Copied the output of --help messages from configure, master, and
  slave.
- The option tables are now in html format.
- Removed the explanation of the default configure flags.

Review: https://reviews.apache.org/r/27556
Only push the tag when tagging a release.

Git prefers branches to tags so be explicit.
Allow mesos containerizer to destroy correctly at each state.