Fix 'td import:create' with import-only permission

Changes:
* Remove client validation for db and table to avoid incurring
  in a failure because an API key that has import only permissions
  to a database cannot list tables. The validation on whether
  the table exists is deferred to the APIs (server side validation)

NOTE: creating a bulk import session with a write-only
      API key is currently not possible

First part of CLT-330