Add a default to file-driver when unspecified over RAPI

Add a default to file-driver when unspecified over RAPI

The file-driver value, used by file-based instances, had a default value when
an instance was being created over the CLI, but not when the instance was
created through the RAPI.

This patch introduces a default value for the remote API and, while doing so,
also centralizes in a single place the definition of such a value, for both the
interfaces, in order to remove the possibility for them to be out of sync.

Fixes Issue 571.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>