Implement remoteRef Configuration Option for Use in Document Pull

[lb4368]

Problem description
For development/debugging purposes it would be useful to be able to specify to check out a gerrit patchset as part of an 'airshipctl document pull' configuration. There is a remoteRef attribute available in the repository checkout section of the airship configuration file that is intended to allow specification of a git reference to be checked out as part of the document pull. However, this option is currently not implemented.

Proposed change
Update the airshipctl document pull functionality to checkout the git reference specified in the remoteRef configuration option if it is specified.

[manojkva]

Please assign this to me. Thanks

[manojkva]

Had a discussion with Ruslan Aliev and here are his comments
"that's a complicated issue, talks about supporting remoteRef were started 1.5 years ago and still there is no solution. there are limitations and complication in go-git module, there is need to deep dive into go-git api, gerrit refs and so on, I can't give any clues unfortunately, it requires a complete research, if it were easy this functionality would have been implemented long ago"

I think this will need a design discussion and would need research.