add support for jinja do statements, add unit test

[thetoolsmith]

Proposed Commit Message

Adding support for jinja do statements in template rendering.

summary: adding support for jinja 'do' statements in jinja templating

Support for jinja do statements will prevent having to workaround the following exception:
jinja2.exceptions.TemplateSyntaxError: Encountered unknown tag 'do'.

LP: #1962759

Additional Context

Test Steps

When processing the following example, the jinja exception is thrown.

{% set data_result = [] %}
{% set data_input = [1,2,3] %}
{% for i in data_input %}
{% do data_result.append(i) %}
{% endfor %}
{{data_result}}

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[blackboxsw]

Thanks for the bug and quick PR @thetoolsmith. I'll await confirmation on the CLA signing
which should become available to me tomorrow.

Confirmed "do" extension is present and functions properly without error on Ubuntu Bionic 18.04 (python3-jinja2 v. 2.10) and later.

Simple test with this changset generates the updated list output without printing the None return value from the append function call"

$ cloud-init query --format "{% set r = [] %}{% set input = [1,2,3] %}{% for i in input %}{% do r.append(i) %}{% endfor %}{{r}}"
[1, 2, 3]

[blackboxsw]

I confirm @thetoolsmith has signed the Contributor License Agreement for cloud-init, welcome to the project and thanks for your submission.