Add first custom Ansible filter

[Alex-Welsh]

Adding a custom ansible filter.

This is mainly to serve as an example that we can extend in the future with more custom filters so we can avoid some of the hairy jinja templating we've needed in the past. (See etc/kayobe/inventory/group_vars/all/firewall for example)

[Alex-Welsh]

This is directly compatible with #1744

[dougszumski]

Maybe I've missed something, but shouldn't the filter live in kayobe? There is already a test framework there too.

[dougszumski]

Any filters designed for Kayobe config would best live in Kayobe, so they can be used there, and in OpenStack Kayobe config etc.

[Alex-Welsh]

Any filters designed for Kayobe config would best live in Kayobe, so they can be used there, and in OpenStack Kayobe config etc.

I agree that this filter might also be useful in Kayobe upstream, though I don't think that would apply to all custom filters. The idea that started this in the first place was that @GavinHeff is writing a custom fact-gathering playbook and there's some very specific templating. A custom filter would be very useful for formating that data, but would be pretty useless upstream.

The main reason I'm adding this is just so that we have a precedent for future filters that we would only care about downstream. The actual content of the filter isn't really the point of the change. Personally, I'd like to keep this downstream for now, and move it upstream if we have a need for it in the future.