Use existing secret for peerdb password, peerdbUI password and nextauth key

[PurseChicken]

It would be great if we could use an existing secret for the following areas:

peerdb password
peerdbUI password
peerdbUI Nextauth

Instead of having to store passwords \ keys in the values file.

[PurseChicken]

@iamKunalGupta Could we get this in? Or do you need me to create a PR for this work?

I think this also addresses #23

[PurseChicken]

I created a PR for this work and have validated it passes helm standards.