add early error message if docker is missing

[ArberSephirotheca]

Describe your changes

src/soopervisor/commons: docker.py

Add new method validate_installation(), which takes Commander instance as argument. For backends using Docker, validate_installation() will be called at the begining of export() method.

src/soopervisor: airflow/export.py, argo/export.py, aws/batch.py , kubeflow/export.py

Invoke validate_installation() at the begining of export() method.

Issue ticket number and link

Closes #72

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I have added thorough tests (when necessary).
• [x] I have added the right documentation (when needed). Product update? If yes, write one line about this update.

[edublancas]

closing due to inactivity