Added External Containerizer documentation.

Adds a markdown document describing the ExternalContainerizer.

Review: https://reviews.apache.org/r/22169/
MESOS-2038: removed dead code from _runTask().

Review: https://reviews.apache.org/r/27567
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.
Updated scheduler driver to exponentially backoff during registration
retries.

Review: https://reviews.apache.org/r/27315
Document authentication prerequisites in docs/release-guide.md.

Review: https://reviews.apache.org/r/27707
Fixed Authenticator SASL auxiliary memory access.

Review: https://reviews.apache.org/r/27741
Added slave metrics for total and used resources.

Review: https://reviews.apache.org/r/27759
Fixed invalid characters in external-containerizer.md

Review: https://reviews.apache.org/r/27724
Abstracted out invoking 'mesos-fetcher'.

Manually rebasing and re-editing https://reviews.apache.org/r/21233/,
which is supposed to be replaced now by this patch.

Original description: "To test the mesos-fetcher (and the setting of
the environment) more cleanly I did some refactoring into a 'fetcher'
namespace."

Also moved fetcher environment tests to fetcher test file. Added two
fetcher tests.

Review: https://reviews.apache.org/r/27516
Fixed MesosContainerizerExecuteTest.IoRedirection test.

Review: https://reviews.apache.org/r/27765
Add destroy tests for docker containerizer.