Allow using existing imagePullSecrets

[sbrunk]

Right now it's possible to specify credentials for a private docker registry via hub.imagePullSecret and singleuser.imagePullSecret. This will create a new Secret from the given credentials and use it for pulling images.

In our setup we already have an existing Secret in our cluster we'd like to use as imagePullSecret. I'd be happy to create a PR adding this but I'd like to discuss how this could be configured first.

Here's the current block in values.yaml for specifying an pull secret (one for the hub and one for singleuser).

  imagePullSecret:
    enabled: false
    registry:
    username:
    email:
    password:

I guess one way would be to add an existing field and then set the pull secrets accordingly if enabled is false and existing is set to some value. What do you think?

[LaurentGoderre]

I opened #1426 for this because we need this too!

[minrk]

closed by #1426