Feat: Additional existing secrets

[m4rg4sh]

Fixes: #368 #283 #370

This PR adresses several issues.

• It fixes the regression described in Wrong Secret Keys for Redis and DB passwords #370 that breaks installation if existing secret refs are not used as redis and database passwords by changing the default keys.
• It adds the possibility to also supply the NAUTOBOT_SECRET_KEY with a existing secret as requested in Add ability to provide secretKey via reference #283. The change should be backwards compatible by still allowing .Values.nautobot.secretKey to be specified but it also introduces a new structure named .Values.nautobot.django.* which takes priority. It still allows the user to specify a key in the configuration or, alternatively, to pass in a reference to an existing secret.
• It also adds the possibility to pass in the NAUTOBOT_SUPERUSER_PASSWORD and _API_TOKEN in the same manner.

All these references can use a user-supplied secret and user-supplied key if necessary but do fall back to the defaults when necessary.
If no references or secret values are supplied the bitnami common chart is used to generate them and populate these secrets.
The superuser and secret key values are by default added to the existing *-env secret. But if referenced individually they are added as independet env variables to the relevant pods. If all of these variables are referenced to an existing secret the *-env secret becomes empty. However it is still created without any data to avoid broken secret references in the other objects.

If all of these features are used together it allows the user to not specify any secrets in the values.yaml file.

[m4rg4sh]

Hey @ubajze could you check this out, please? It's related to the previous PRs by @cardoe and myself.

[ubajze]

@m4rg4sh, I'm sorry I haven't been more responsive, but I've been quite busy lately. I will try to schedule a time on Monday to check these PRs. Then we will have to prepare a new release to incorporate all these changes.

[ubajze]

@m4rg4sh I went through the PR. I think it all makes sense.