Plugins: Add support for anonymous access

Upgrade pygerrit2 to 2.0.12 and add the --anonymous option to the
plugins.py script.

The --anonymous option is mutually exclusive with the --netrc option.

Mention these authentication options in the publishing documentation.

Bug: Issue 12504
Change-Id: Iff594e84a7817ce8689ad45e7ed8e37e3f8bf72c
3 files changed