Container Support

Problem: BaseDistribution.validate() is too Distribution specific

Solution: Move validate() to the Distribution instead

This patch allows classes that inherit from BaseDistribution to call super().validate(data) to get the validation provided by the ModelSerializer.

This patch also removes base_path from BaseDistribution because it is defined on the Distribution. DockerDistribution also defines it for itself.

re: #4669 https://pulp.plan.io/issues/4669

Problem: Publications are not needed for Docker plugin

Solution: Add 'repository_version' field to DockerDistribution

This patch allows users to create DockerDistributions that serve repository versions. A distribution can be created with either a repository or repository version defined. If a repository is defined, the latest version of that repository is always available at the distribution. If a repository version is specified, then that repository version is always available at that distribution.

This patch updates an existing test and adds a new one. The two tests assert that a docker or podman pull can be performed when a repository is used in a distribution and when a repository version is used.

This patch also fixes the tests to work with docker as the client.

closes: #4669 https://pulp.plan.io/issues/4669

Problem: DockerDistribution name field not validated correctly

Solution: Define the name field on the DockerDistribution serializer

The name field from the BaseDistribution serializer validates that the name doesn't collide with any other Distributions.

closes: #4600 https://pulp.plan.io/issues/4600

Problem: DockerDistribution name field not validated correctly

Solution: Define the name field on the DockerDistribution serializer

The name field from the BaseDistribution serializer validates that the name doesn't collide with any other Distributions.

closes: #4600 https://pulp.plan.io/issues/4600

Problem: DockerDistribution name field not validated correctly

Solution: Define the name field on the DockerDistribution serializer

The name field from the BaseDistribution serializer validates that the name doesn't collide with any other Distributions.

closes: #4600 https://pulp.plan.io/issues/4600

Problem: DockerDistribution name field not validated correctly

Solution: Define the name field on the DockerDistribution serializer

The name field from the BaseDistribution serializer validates that the name doesn't collide with any other Distributions.

closes: #4600 https://pulp.plan.io/issues/4600