Add MultiRemoteDistribution support for pull-through caching with multiple remotes

[dkliban]

Problem Statement

Currently a distribution can only point to a single remote for pull-through caching. There are use cases where a distribution needs to pull content from multiple upstream sources. A new MultiRemoteDistribution is needed that can point to multiple remotes. When the content app handles a pull-through cache request using such a distribution, it should try each remote in order until it finds one that has the requested artifact. Once found, the content should be streamed to the client and cached in Pulp.

Requirements

• Add a new MultiRemoteDistribution model that can reference multiple remotes
• When serving a pull-through cache request, the content app must try each remote associated with the distribution until the artifact is found
• Once a remote with the content is found, stream the artifact to the client and cache the package in Pulp
• Each plugin must be able to inherit from MultiRemoteDistribution and implement it for its specific content type