Add filesystem abstraction

[tcaiazzi]

We need to add a filesystem abstraction to the Lab and Machine classes to decouple the execution of Kathará from having a path on the host filesystem.

This will enable users to use Kathará API to fully configure a network scenario.

We plan to use the PyFilesystem2 library to add a fs variable representing the filesystem abstraction to Lab and Machine classes.

[Skazza94]

Temptative Roadmap:

• Introduce pyfilesystem into Lab and Machine classes
• Lab: if constructor is called with path parameter, it means that it has a host folder in the OS filesystem (use osfs://). Otherwise, create an in-memory folder using mem:// filesystem.
• Change Lab class to support new pyfilesystem reference, stored in fs.
• Change Machine to reference the device folder by using the Lab.fs attribute, stored in fs.
• Change the Machine.pack_data in order to create the tar file from the Machine.fs attribute.
• Add a mixin FilesystemMixin that is inherited by Lab and Machine.
• Add utility methods to FilesystemMixin to add a file in the filesystem reference:
  • Create FilesystemMixin.create_from_string(value: str, dst_path: str) that creates a file in the specified full destination path, with the content of the value parameter.
  • Create FilesystemMixin.create_from_path(src_path: str, dst_path: str) that creates a file in the specified full destination path, copying the content of the file in src_path.
  • Create FilesystemMixin.create_from_stream(stream: IO, dst_path: str) that creates a file in the specified full destination path, writing the content of the stream parameter.
• Remove shared_startup_filename and shared_shutdown_filename from Lab constructor and recompute them everytime, so that if the file is added later it will be correctly uploaded.
• Remove startup_filename and shutdown_filename from Machine constructor and recompute them everytime, so that if the file is added later it will be correctly uploaded.
• Add docs strings to FilesystemMixin class.
• Add unit tests for FilesystemMixin class.