Currently deckhand render validation is disabled by default with no
option to override that behavior from the command line. Resolve this
by:
1. Adding CLI render flag 'validate', default=True
2. Updating CLI documentation
3. Update pegleg.engine.site.render method to include configurable
validate flag
4. Update pegleg.engine.util.deckhand.deckhand_render method to
validate=True by default (previously False)
5. Update pegleg.engine.util.deckhand.deckhand_render method to
perform deckhand's validate all function on rendered documents
NOTE: Validation logic is handled in deckhand, see
https://opendev.org/airship/deckhand/src/branch/master/deckhand/engine/layering.pyhttps://opendev.org/airship/deckhand/src/branch/master/deckhand/engine/document_validation.py
Change-Id: I042fad4b2bf08c88e3a2eef6a54dede5d45c28f5