[AIRFLOW-442] SFTPHook

[sdiazb]

Dear Airflow maintainers,

Please accept this PR.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-442

Description

• SFTP Hook.

Tests

• My PR adds the following unit tests (interacting with the Travis OpenSSH Server):
• https://github.com/sdiazb/airflow/blob/master/tests/contrib/hooks/test_sftp_hook.py

[codecov-io]

Codecov Report

Merging #2465 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2465      +/-   ##
==========================================
+ Coverage   69.39%   69.43%   +0.03%     
==========================================
  Files         146      146              
  Lines       11349    11350       +1     
==========================================
+ Hits         7876     7881       +5     
+ Misses       3473     3469       -4

Impacted Files	Coverage Δ
airflow/utils/db.py	84.55% <100%> (+0.12%)	⬆️
airflow/jobs.py	77.12% <0%> (+0.38%)	⬆️

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[bolkedebruin]

Hello @sdiazb, if you want use to review this contribution please make sure it is clean and squashed and adheres to the commit guidelines.

[sdiazb]

Thanks @bolkedebruin, I'll close this one and create another following the commit guidelines and squashing the commits.