Update examples in documentation for env lookup plugin

[jtyr]

SUMMARY

The current env lookup plugin is setting the default value to empty string if the variable is not defined. This is nor particularly great as it doesn't allow to use the default() filter if the variable is not defined. It would be great if we could change the default value from empty string to None but that would break the backward compatibility. This PR is keeping the current behavior (non-existing variable returns empty string) and it's adding the possibility to define custom default value. It also adds some basic unit tests.

ISSUE TYPE

• Feature Pull Request

COMPONENT NAME

env lookup plugin

ADDITIONAL INFORMATION

# This returns empty string
lookup('env', 'MYVAR')

# This returns the default value defined in the lookup plugin
lookup('env', 'MYVAR', default='myval')

# It also allows to use the default() filter if setting the default to None
lookup('env', 'MYVAR', default=None) | default('myval')

[flowerysong]

You could instead use | default('myval', True), which returns the default for false values in addition to undefined.

[samdoran]

Rather than a change in behavior, we should add an example to the documentation of the solution @flowerysong suggested.

[jtyr]

Adding this new feature doesn't change the behavior. It just allows to set default value using one filter. But I agree, that documenting the | default('myval', True) would be a good thing as well.

[sivel]

The consensus amongst all of the core devs today, was that this feature would not be accepted. Replacing this feature with a docs pull request would be.

[jtyr]

OK. Let me amend the PR to document the use of the default() filter and adding the tests for the existing functionality of the lookup plugin.

[jtyr]

@samdoran @sivel Please see the latest commit where I have removed my original implementation of the default value and just documented how to set default value via the default filter.

To be honest, I'm not happy about such implementation as it doesn't allow to differentiate between env variable which is not defined and env variable which is defined but has empty string as its value. This is what the original PR was facilitating.