[AIRFLOW-5665] Add path_exists method to SFTPHook

[TobKed]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5665
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal (AIP).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Description

• Here are some details about my PR, including screenshots of any UI changes:

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

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 (not including Jira issue reference)
  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"

Documentation

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  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[mik-laj]

I restarted travis jobs.

[codecov-io]

Codecov Report

Merging #6344 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #6344      +/-   ##
=========================================
- Coverage   80.41%   80.1%   -0.31%     
=========================================
  Files         616     616              
  Lines       35808   35811       +3     
=========================================
- Hits        28794   28686     -108     
- Misses       7014    7125     +111

Impacted Files	Coverage Δ
airflow/contrib/hooks/sftp_hook.py	93.82% <100%> (+0.23%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️

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