Respect stdout callback being set

[willthames]

If stdout callback is set, either through config or environment
variable, it should be respected, not overridden.

[willthames]

This seems to work if the environment variable is set (as expected) but not if stdout_callback is set in ansible.cfg

It's not clear to me if there's a mechanism to respect ansible.cfg for this variable, but it's an acceptable failure for me.

[ansible-zuul]

Build succeeded.

• ansible-tox-py36 : SUCCESS in 9m 14s
• ansible-tox-py37 : SUCCESS in 9m 16s
• ansible-build-python-tarball : SUCCESS in 3m 57s
• ansible-tox-linters : SUCCESS in 5m 36s
• ansible-tox-py27 : SUCCESS in 8m 06s

[matburt]

This does have the effect of short circuiting some of the capability of Runner for gathering this information as part of its event payloads. I wonder if we should wrap this up into a larger effort to clarify support for ansible.cfg.

[willthames]

It's not just not supporting ansible.cfg though - there has to be some way to override the stdout callback - you can't possibly predict that people will want only either one of the two built in stdout callbacks

[wenottingham]

see #387