[AIRFLOW-1509] SFTP Sensor

[sdiazb]

Dear Airflow maintainers,

Please accept this PR.

JIRA

• My PR addresses the following AIRFLOW-1509 JIRA issue and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-1509

Description

• SFTP Sensor.
  This PR leverages the SFTPHook created in AIRFLOW-442 that should be merged before this PR.

Tests

• My PR adds the following unit tests:
  https://github.com/sdiazb/airflow/blob/sftp_sensor/tests/contrib/sensors/test_sftp_sensor.py

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
  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"

[sdiazb]

@Fokko I've opened again the old PR to force the build. I think everything is ready to be merged now. Many thanks for your help.

[Fokko]

LGTM, Thanks @sdiazb for adding the tests.