Add `yamllint` to the project

[xmkg]

since the project is using many YAML files, it would be beneficial to use a linter both in tox testing and CI. This patch adds yamllint 1.35.1 to the project as a test dependency. this patch also adds a new tox target named yamllint which runs yamllint at the which runs yamllint at the hotsos/ and tests/ directories.

the yamllint currently uses the yamllintrc file supplied in this patch. this configuration inherits from the default configuration of the yamllint, and excludes only one path, i.e.: fake_data_root, because it contains test data captured from some environment and should be kept as-is. For further reference regarding available options, please see the docs.

summary of changes made to yaml files:

• changed all truth values (True, False) to (true, false)
• added linter suppression comments for false positives
• removed all trailing spaces, except for test file contents
• removed all redundant empty lines from EOF
• fixed indentation issues reported by the linter

PS: please don't be scared of the gargantuan list of YAML file changes, these are all just cosmetic changes to make the linter happy.

[xmkg]

Removing the CI step for now, because it completely removes all tasks from the "tests.yaml" file since the file has been changed. This raises another question: "Should we separate ".yaml" files so newly added CI steps do not interfere with the execution of the existing ones?"

[xmkg]

I missed reverting expr changes on these:

• src_migration.yaml
• lp1948906.yaml
• lp1996230.yaml
• lp2029857.yaml

And there are some unintended changes to tests/unit/fake_data_root which are remnants of before "ignore" path.

Fixed.