Walk back Python 3 changes, use python2 explicitly.

None of our automatic build environments have Python 3 currently.
Fix glob that found no input files.
Support both python2 and python3.
Load balancing example should use new syntax.

One of the examples was still referring to the old way of defining
directors, which I am sure is confusing for the new users this
document is meant for.

Noticed by: coredump on irc.

Used the occation to do some minor scrubbing of the text.
Use consistent naming through the example.
Truncate %D varnishncsa format specifier to an integer value.

This follows the format Apache logs in.
Exclude vcl_var since it is included in vcl.rst.
Add missing returns, and do a bit of cleanup