ssg/rule.yml does not block rules which are missing the `documentation_complete` tag.

#### Description of problem:
ACS-ICS team noticed that there were rules being pulled into profiles that were missing the `documentation_complete` tag. Investigation lead us to find that the `ssg/yaml.py` file (line 115), is designed such that it treats rule files without the tag as `documentation_complete:true`, which can lead to problems like: https://redhat.atlassian.net/browse/CMP-4339

#### Actual Results:
A rule.yml file that has no `documentation_complete` tag is treat as `documentation_complete:true` during build.

#### Expected Results:
A rule.yml file that has no `documentation_complete` tag should cause the build to fail or maybe issue some warning?

Start at ssg/yaml.py around line 115 and trace how a rule.yml without documentation_complete is interpreted during the build. Check the existing build validation or warning tests, then define and verify behavior for a missing tag based on whether the build should fail or warn.

Written by the indexing model from the issue text.