Add support for --load-yml to cci task and cci flow commands

[jlantz]

The BaseCumulusCI runtime has long had support for passing additional_yaml which is then added to the yaml merge stack. This functionality was used extensively in MetaCI to inject build specific configuration overrides for certain build plans. However, it was never exposed to the cci cli.

This PR adds a new --load-yml <path/to/additional.yml> option flag for all subcommands of cci task and cci flow by adding support for the option in the main command and then implementing it as an option for the cci task and cci flow commands.

This change should have no impact on existing users and is purely additive functionality that didn't exist before. I've added a docs section to the config docs explaining the use case which is included with this PR along with full test coverage.

[jstvz]

Fixes #3429.

[jstvz]

@jlantz Following up on this PR.

[jlantz]

I will get those two changes done and submitted this week, along with a whole bunch of new stuff to put in the review queue ;)