[CI] Define git author and commiter information as environment variable

[serban-nicusor-toptal]

Submission Checklist

• [ ] Run unit tests: ./runTests.py src/test/unit
• [ ] Run cpplint: make cpplint
• [ ] Declare copyright holder and open-source license: see below

Summary

Fix an issue in CI where commits (mostly merges) from Jenkins are coming from a default user ( default git plugin author ). See https://github.com/stan-dev/stan/pull/3153 as an example.

Intended Effect

How to Verify

Side Effects

Documentation

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)