Change default registry for new bundles from getporter to localhost:5000

[carolynvs]

I think it would be helpful if our default registry was one that people could conceivably push to. Right now it defaults to "getporter" which no one (even me) should push to. By setting it to a local test registry, we can provide more stream lined instructions, by having people run a local registry to try out porter without needing an account somewhere for example.

This change should be made on the release/v1 branch.

Steps

1. Edit our new bundle template and change the registry from getporter to "localhost:5000". https://github.com/getporter/porter/blob/release/v1/pkg/templates/templates/create/porter.yaml
2. Let's change the comment above that line to explain what the registry field is, instead of telling them to change it. For example, "registry where the bundle is published to by default".

You can test this out by following the steps from the contributor tutorial to build your changes. Use the porter binary that you built to make a new bundle and verify that your change shows up in the generated porter.yaml when you run porter created.