Support for volumes in AWS and GCP

[bharatak]

When trying to mount a volume (disk in GCP) to the docker image and installation of app aborts with the following error

Directory /home/dataiku/dss already exists, but is not empty. Aborting !

The current logic in the wrapper script which is run as entry point to the Dataiku-provided Docker image https://github.com/dataiku/dataiku-tools/blob/master/dss-docker/run.sh looks at the provided data directory for a well-known DSS file in order to automatically determine whether it should install a new instance or upgrade an existing one, as a convenience for an easier use. It should be possible to change that in a production environment to adapt to use a EBS volume or a GCP disk.