Add Git based Index

[r0mainK]

Pretty big PR implementing the Git based index:

Breakdown of the changes:

• [x] main.py: added functions for git index args (remote_repo, username, password) and template args template_model and template_readme; add --force to initialization parser so we can not destroy registry carelessely, added --meta keyword to publish parser in order to point towards a JSON that will contain all addtionnal information we may want to give (extras, etc). It is not necessary, I added a default JSON template to the repo that is mostly empty if the flag is not used.
• [x] delete.py/dump.py: moved code to registry.py, files were deleted
• [x] gcs_backend.py: took out fetch_index and update_index, added a helper function to create the GCS client since both connectand the new method initialize_backend use it and added/renamed some errors
• [x] model.py: small change, since backend and index are now separate I had to change a bit the logic to still make it possible to query with only the UUID, added an index dict param to the load function
• [x] models.py: same story, added it in the constructor of GenericModel
• [x] registry.py: a lot of changes, however most of the logic is not here anymore, the functions almost only call backend/git_index methods. notable change: listdoes not require backend anymore
• [x] storage_backend.py: took out fetch_index and update_index, added initialize_backend and errors
• [x] templates/*: contains the mebtionned above JSON template as well as jinja2 templates from src-d/models
• [x] setup.py/requirements.txt: added src-d/models libs and line to add templates to the package
• [x] index.py: new file that contains the GitIndexclass now used to maintain the index. Incorporates the old functions of src-d/models as well as methods to fetch/upload the index, add/remove models, and initialize registries

[vmarkovtsev]

OK, this is a bit too much for me :)

Please split. I would suggest:

1. Readme and docs are the last
2. Templates and requirements are the first
3. storage_backend.py changes separate
4. The rest shall be splitted too - see what's the best way yourself

Amazing work btw, can't wait to have it merged.

[r0mainK]

Aha no problem, thks ^^ okay then I'll split in something like 5 or 6 PRs, then close this

[marnovo]

@r0mainK @vmarkovtsev what issue is this related to?

[vmarkovtsev]

@marnovo https://github.com/src-d/backlog/issues/1205

[r0mainK]

@marnovo this issue: https://github.com/src-d/backlog/issues/1205

basically having the index of the registry on the backend was not great so now it's gonna be exclusively on github, e.g. https://github.com/src-d/models , and itll be updated with modelforge cmds.

[vmarkovtsev]

Closing - it was splitted.

[vmarkovtsev]

LOL

[r0mainK]

aha

[marnovo]

Thanks! Please let's always try to reference things around so people have more context of why/what is going one.

Idea for the future: intelligent PR splitter, to split large PRs into more manageable ones for review. ;)