[AIRFLOW-6510] - Fix Druid operator rendering from file

[alexnu]

Dear maintainers,

Please accept my PR which fixes the way druid specification json is loaded by leveraging template fields as in most other operators, rather that manually parsing the file in the operator.

Fixed existing tests and deleted one unneeded test case, as the operator is no longer opening a file.

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Issue link: AIRFLOW-6510

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2754427). Click here to learn what that means.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master    #7102   +/-   ##
=========================================
  Coverage          ?   84.95%           
=========================================
  Files             ?      682           
  Lines             ?    39043           
  Branches          ?        0           
=========================================
  Hits              ?    33168           
  Misses            ?     5875           
  Partials          ?        0

Impacted Files	Coverage Δ
airflow/contrib/operators/druid_operator.py	82.35% <50%> (ø)

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