do-release-tags: Add option to pull commits from remote repository

[ajodock]

Currently do-release-tags implements the "binary promotion model", but it could be extended to help users with the "promoting content between repositories" method as well.

The user would simply need to add a remote to their ostree repository:

ostree --repo=/repo remote add remote-name http://myremote.my.domain/repo-dev

And then when running the do-release-tags script a remote would have to be specified to pull missing commits from. Something like:

./do-release-tags --repo /repo --releases release.yml  --remote remote-name

Not only is this useful for promoting your own content, but can also be used to help automate mirroring and promotion of content from third party repositories (i.e. creating local staging/production of CentOS Atomic images).