Require the variable key to be set and unique

See merge request !234

Add missing stage when doing retry on a build

See merge request !236

Make the YAML validation more stricter

- deny all globals that are not hashes and doesn't contain script, the
  script is required now
- verify that script is either string or array of an strings

See merge request !235

- deny all globals that are not hases and doesn't contain script, the script is required now
- verify that script is either string or array of an strings