Repos and branches individual overrides

[adriangibanelbtactic]

Fixes #906 .

Thanks to this pull request you can now override individual repos urls and branches.
There's no longer need to clone/fork all of the ONLYOFFICE repos for just testing a single change in a single repo.

Usage:

cd tools/linux
python3 ./automate.py \
      --branch=tags/v9.0.4.52 \
      --repo-overrides=server=https://github.com/GITHUBUSER/server.git,web-apps=https://github.com/GITHUBUSER/web-apps.git \
      --repo-branch-overrides=server=tags/v9.0.4.52-change1,web-apps=tags/v9.0.4.52-change1

Feedback is welcome

I am very interested on adding this feature. So, please, Ascensio, give me feedback. I am willing to change this PR as much as needed so that it fits your programming/documentation style so that this can be included upstream.

Thank you!