[utils] Create a base class for Storage and CodeStorage

They share some logic and use so avoid code duplication and
hidden missing pieces.

Fixes #23
3 jobs for 23_clean_storage_classes in 9 minutes and 29 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed #164600
docker
build_linux_36

00:03:52

73.0%
passed #164619
macosx
build_macosx_36

00:05:37

73.0%
failed #164601
macosx
build_macosx_36

01:30:01