[AIRFLOW-55] Add support for HDFS remote logging

[dhuang]

Implemented using WebHDFS hook because HDFS hook (using snakebite)
does not support a copyFromLocal.

Please accept this PR that addresses the following issues:

• https://issues.apache.org/jira/browse/AIRFLOW-55

Testing Done:

• Added unit tests to test the path parsing.
• Was able to view logs in the UI that were pulled from HDFS.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3c450fb). Click here to learn what that means.
The diff coverage is 16.66%.

@@            Coverage Diff            @@
##             master    #2119   +/-   ##
=========================================
  Coverage          ?   67.98%           
=========================================
  Files             ?      142           
  Lines             ?    11005           
  Branches          ?        0           
=========================================
  Hits              ?     7482           
  Misses            ?     3523           
  Partials          ?        0

Impacted Files	Coverage Δ
airflow/configuration.py	83.73% <ø> (ø)
airflow/www/views.py	67.87% <0%> (ø)
airflow/utils/logging.py	23.3% <16.27%> (ø)
airflow/bin/cli.py	51.83% <50%> (ø)

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[Fokko]

Nice one, thanks!