Fix for issue #7121 - Changed py renderer global variables to be consistent with pydsl

As per issue #7121 - this PR changes the global variable names salt, grains, opts, pillar, env and sls to be __salt__, __grains__, __opts__, __pillar__, __env__ and __sls__ to restore consistency between the py and pydsl renderers.

Simple change, but it will break compatibility with existing py rendered states/pillars that utilise the old global variable names.