[Feature] Make better use of cornice/colander validators

[fmigneault]

Description

Using the following definitions:

• https://cornice.readthedocs.io/en/latest/validation.html
• https://cornice.readthedocs.io/en/latest/schema.html#using-colander

It is possible to make better use of Cornice utilities to perform pre/post validations instead of manual try/except with raise of the HTTPException.

• [Feature] Improve schema error reporting and language translation #112
  (see https://cornice.readthedocs.io/en/latest/i18n.html)

• [Feature] HTML format #210

• support YAML representation of the process using Accept header or f/format query #456

• use content_type in cornice decorator over request view to validate the list of permitted content types in request body

  • add 415 Unsupported Media Type to responses corresponding to content_type validation
    (should be auto-handled by add_cornice_service handler, but test to validate)

• use from cornice.validators import DEFAULT_VALIDATORS, colander_validator, colander_body_validator, colander_headers_validator, colander_path_validator, colander_querystring_validator to validate items referred by schema
  (see also Question: Ambiguity regarding multi-validator use case with colander Cornices/cornice#587)

• use accept and let cornice/colander perform the 406 Not Acceptable check
  (consider https://cornice.readthedocs.io/en/latest/validation.html#using-callables-1 to handle f instead of accept - inject ihe header?)

• Simplify exception definitions #215
  return contents for corresponding errors (ie: format the JSON with expected OGC structure) using custom error_handler
  (see https://cornice.readthedocs.io/en/latest/validation.html#dealing-with-errors)