Move space reporting constant to constants.py

Move space reporting constant to constants.py

This patch moves the constant which is used to determine
whether a storage type provides storage space reporting
from the utils package to the constants. This way, we
can also use it in haskell and it fits there semantically
better anyway.

Additionally, I added unit tests that make sure that the
constant is in sync with the actual implementation in the
backend. This will make sure that when new storage types
provide space reporting, those functions are actually used
in the backend.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>