title: use airflow variables in BashOperator dag
body: <p>I try to install the python requirements with following Dag</p>
  
  <pre><code>import airflow
  from datetime import datetime, timedelta
  from airflow.operators.bash_operator import BashOperator
  import logging
  
  
  args = {
      'owner': 'airflow',
      'start_date': airflow.utils.dates.days_ago(1),
      'provide_context': True
  }
  
  
  dag = airflow.DAG(
      'set_up_python',
      schedule_interval="@once",
      default_args=args,
      max_active_runs=1)
  
  t1 = BashOperator(
          task_id= 'echo_varconf_path',
          bash_command="echo {{ var.conf_path }}",
          dag=dag
     )
  
  t2 = BashOperator(
          task_id= 'install_python_requirements',
          bash_command="pip install {{ var.conf_path }}/requirements.txt",
          dag=dag
     )
  </code></pre>
  
  <p>Prior I set the variable conf_path however I can't reach the variable in the bash command. In the logs it seems that the {{ var.conf_path }} part is simply ignored: </p>
  
  <blockquote>
    <p>[2018-03-29 23:56:25,858] {{base_task_runner.py:98}} INFO - Subtask:
    [2018-03-29 23:56:25,858] {{bash_operator.py:88}} INFO - Running
    command: pip install /requirements.txt [2018-03-29 23:56:25,866]
    {{base_task_runner.py:98}} INFO - Subtask: [2018-03-29 23:56:25,866]
    {{bash_operator.py:97}} INFO - Output: [2018-03-29 23:56:26,688]
    {{base_task_runner.py:98}} INFO - Subtask: [2018-03-29 23:56:26,687]
    {{bash_operator.py:101}} INFO - Invalid requirement:
    '/requirements.txt'</p>
  </blockquote>
  
  <p>As you see in my dag definition I try to output the variable in a first task, the output of this one is empty too:</p>
  
  <blockquote>
    <p>[2018-03-29 23:56:25,856] {{base_task_runner.py:98}} INFO - Subtask:
    [2018-03-29 23:56:25,856] {{bash_operator.py:88}} INFO - Running
    command: echo  [2018-03-29 23:56:25,866] {{base_task_runner.py:98}}
    INFO - Subtask: [2018-03-29 23:56:25,865] {{bash_operator.py:97}} INFO
    - Output:</p>
  </blockquote>