Download ISO in more simple way.

In order to add support of transferring an ISO from one datastore
to another, below changes are done:
- Return and close connection for read_connection
- Add FileReadHandle similar to FileWriteHandle
- Add common headers to _create_connection
- Accept pre-build cookie for FileReadHandle/FileWriteHandle
- DatastoreUrl as parameter for FileReadHandle/FileWriteHandle

Closes-bug: #1975618
Change-Id: I311c98201e3a89db561b7a0c64592803b32a8b31