Late-bind variable interpolation via YAML tag

[jgraichen]

Late-bind variable interpolation should be marked in the YAML file and finally handled by the tower e.g.:

# jinja | yamlet

data:
  var: 5
  result: !format "some late bound key with {data.val}"

This is much safer than late bound formatting the whole pillar but requires extensive interaction between the yamlet renderer and the salt tower.