Use a secret instead of a configmap for alpha config

[mkjpryor]

This PR addresses the concerns in #104, namely that using the alpha config requires storing the client secret in a configmap, by moving the whole alpha config to a secret.

Once we have a better way of locating secrets, this can be changed.

[mkjpryor]

Note that the existing behaviour of alphaConfig.existingConfig pointing to an existing configmap is retained for backwards compatibility.

[mkjpryor]

Note that the patch version was bumped rather than the minor version as:

1. The change is backwards compatible
2. I consider storing secret information in a configmap to be a bug

[pierluigilenoci]

@JoelSpeed, could you please take a look at this?

[pierluigilenoci]

@mkjpryor, could you please rebase?

[mkjpryor]

@pierluigilenoci rebase done

[jessebot]

Thanks for your work here! Could I ask if there's any blockers here? This feature would be awesome!

[pierluigilenoci]

@jessebot I was waiting for a review from @JoelSpeed.
Ref: #104 (comment)