New endpoint CreateImportCeph

Depends #1051 (closed)

We need a new endpoint that will allow a user with scope CephImport enabled to create a new DataImport of type Images with a specific workflow (that will use a new task).

The serializer immediately creates a dataimport and requires :

• a bucket name
• a corpus id
• (optionally) an element id in the corpus
• (optionally) a path on the bucket

The command generated would be (with all options set):

python -m arkindex_tasks.import_ceph --bucket=XXX --corpus=YYY --element=ZZZ --path=path/to/whatever

The Ceph settings must be provided as environment variables (endpoint, access keys & region).