Abstract node free disk space check

Both create instance and grow disk check the free disk space on nodes
using the same, duplicate code. Since we'll need this in other places in
the future, we abstract the check into a new function.

The patch adjusts the error message to be more in-line with the one for
memory checking, and fixes the exception raised for RPC errors.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>