Cleaning of unused container resources

[dimitar-dimitrow]

Currently images and their content(manifest, config, layers) could not be cleaned. Once a container is created the image and its content are fetched. They remain cached on the system forever even if the container is removed. This could lead to no space left error upon creating of a container if the disk space is limited.

Acceptance:

• Unused images are automatically cleaned after configurable time.
• The corresponding unit testing is done.
• The relevant user documentation is provided

Tasks:

• Implement cleaning of unused container resources #36
• Provide relevant user doc about cleaning of unused container resources kanto#104
• Unit test the resources clean up implementation #53