vagrant-spk should support listkeys command

[ocdtrekkie]

I might be able to write this, but we need a way to list vagrant-spk's available app keys. Even when I have Sandstorm installed on the same base machine as vagrant-spk, I have no way to do this, since vagrant-spk has chosen to store it's sandstorm-keyring in a separate file location than Sandstorm itself.

[ocdtrekkie]

So, documented here is how to do it, currently, which entails SSH'ing to the VM and running the SPK command... whilst manually specifying the keyring location since it's different than what SPK is used to: https://docs.sandstorm.io/en/latest/developing/publishing-apps/#double-check-your-app-id

The docs here also document doing spk keygen this way, but vagrant-spk keygen was also implemented. I should add vagrant-spk listkeys in the same manner to save people these steps, and then clean up the documentation to use the vagrant-spk versions instead of the ssh & spk versions documented here.