[AIRFLOW-2891] Option to set container_name for DockerOperator

[rocketbitz]

Make sure you have checked all steps below.

Jira

• My PR addresses the following:
  • https://issues.apache.org/jira/browse/AIRFLOW-2891

Description

• Here are some details about my PR, including screenshots of any UI changes:
  • allows specification of docker container name for containers spawned by DockerOperators

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  • I have added to existing tests

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8