change boolean values in config from yes/no to true/false

[arbll]

What does this PR do?

Changes all boolean values in agent 6 config files from yes/no to true/false.

Motivation

Sister of DataDog/datadog-agent#2171
Main goal was to avoid confusion when using environment variable instead of config files as yes/no is not supported in that case.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

[gzussa]

LGTM to me. However, make sure it is safe to do so in the code (good use of is_affirmative)

[l0k0ms]

LGTM 👌

[masci]

Not sure what the reviews mean: did anybody check we're making good use of is_affirmative as @gzussa pointed out? Why approving if we're not sure?

[arbll]

As yes/no and true/false are supposed to be evaluated at the YAML level, instance.get(boolean_key) should return a bool (only doubt I have is with A6 if there is some weirdness with the go/python bridge). Worrying about proper use of is_affirmative only make sense if values are passed as string.

[masci]

Thanks @arbll !