buildstream/plugin.py, doc/source/projectconf.rst: Changed policy for core...

buildstream/plugin.py, doc/source/projectconf.rst: Changed policy for core plugin format revisioning

The policy for format revisioning of core plugins maintained in BuildStream
now changes such that we revision all plugins under the same core format version.

This means that format extensions in plugins should be revisioned in
the global _versions.BST_FORMAT_VERSION along with the core format, this
simplifies parameterization of project.conf for usage of core plugins.

This did not really require any code support as our plugins have never
been revisioned before.