Unfortunately, w/o rewrite of gerrit_util, one can't take advantage
of existing auth.Authenticator (which also doesn't know about ~/.netrc
or .gitcookies, but that's fixable). This rewrite, however, will likely
cause issues for nasty scripts outside of depot_tools which use
gerrit_util as a Python library. So, I followed outdated way of
gerrit_util :(

Also contains refactoring of auth library, which was kept backwards
compatible for the same reasons as above.

R=vadimsh@chromium.org

Test-with: led tool
Test-artifact: https://ci.chromium.org/swarming/task/3cf4687dfd26ca10?server=chromium-swarm.appspot.com
Bug: 834536
Change-Id: I6b84b8b5732aee5d345e2b2ba44f47aaecc0f6c5
Reviewed-on: https://chromium-review.googlesource.com/1018488Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>