Container Support

Motivation:

We're currently sync'ing some very large repos. In reality we're only interested in the particular tags but we're getting everything, which is weighing in at TBs at present and growing with each puddle for each version.

It would be nice to be able to limit the tags we sync.

Solution:

Provide a config on the Remote config called "whitelist_tags". This config will accept comma separated values of tags to sync.

When the "whitelist_tags" option is provided:

Pulp will sync just those tags + manifests associated with the tags and all the corresponding layers. In case of on_deman download policy layers won't be downloaded.

Note: If the list of tags was changed, and tag X is not wanted anymore, a manual removal of that tag X should occur in the local repo( in case repo is not in a mirror mode, mirror is not supported for now)
Note: If some invalid tags provided are invalid, Pulp will only sync the available ones in the remote repo.

When the "whitelist_tags" option is not provided:
Pulp will sync the whole repo.

For pulp2 implementation check https://github.com/pulp/pulp_docker/pull/241