Fixes #4030

Update documentation about Jinja2 expressions in playbooks

Closes #3992

Adding documentation for role default variables

Fixes #4045

This was breaking at least the script module, so it would seem best
to check for it and set it to the default executable value

Currently, the accelerate daemon will be run as the sudo_user if
specified, which is incorrect.

Fix: raise correct error in accelerate mode

fix flattened plugin name in changelog

Also some minor cleanup of some fb* variables and functions, which
were remnants of the old fireball2 name

Fixes #4021

Merge branch 'devel-duplicates2' of https://github.com/smoothify/ansible into smoothify-devel-duplicates2

Merge branch 'doublecallback' of https://github.com/sergevanginderachter/ansible into sergevanginderachter-doublecallback

Merge branch 'devel-duplicates' of https://github.com/smoothify/ansible into smoothify-devel-duplicates

The play was just checking for the presence of the keyword in the
YAML datastructure, and not the value of the field, so doing something
like variable substitution was always causing the play to be accelerated

* also minor tweak to the # of retries in the connection attempt

Fixes #4012

Fixes #3943

Fixes #3556

Conflicts:
	lib/ansible/playbook/__init__.py
	lib/ansible/playbook/play.py

fix JSON extra vars quotation.

Loading the default variables in _build_role_dependencies() lead to
a side-effect where those variables were over-riding inventory variables.