This fixes a few issues,
- ${foo}${bar} would be parsed as a variable named foo}${bar,
  which wouldn't be easily fixed without breaking ${foo.${bar}}
- allows escaping . in variable parts so e.g.
  ${hostvars.{test.example.com}.foo} works

This is slower than using re. 3 million templating calls take about
about twice as long to complete with this compared to the regexp,
from ~65 seconds to ~115 seconds on my laptop.