Add the option to use existing secrets for DB, imagePullSecrets and license file, fixes #61

[mpsOxygen]

This PR adds the following functionality (by default they are disabled):

• the ability to use a preexisting imagePullSecrets thru these values

  imagePullSecrets:
    # -- The base64 encoded dockerconfigjson
    secret: ""
    # -- Use an existing imagePullSecret created by a secrets operator
    existingSecret:
      # -- By default the existing secret is disabled, set this to true in order to provide an existing imagePullSecret
      enabled: false
      # -- The existing imagePullSecret name
      name: ""

• the ability to use a preexisting secret for database connection details thru these values:

    additionalEnv:
      null
      # -- Uncomment these when using secret.existingDbSecret.enabled = true
      # - name: DB_NAME
      #   valueFrom:
      #     secretKeyRef:
      #       name: secret-name
      #       key: secret-key
      # - name: DB_HOST
      #   valueFrom:
      #     secretKeyRef:
      #       name: secret-name
      #       key: secret-key
      # - name: DB_USER
      #   valueFrom:
      #     secretKeyRef:
      #       name: secret-name
      #       key: secret-key
      # - name: DB_PASSWORD
      #   valueFrom:
      #     secretKeyRef:
      #       name: secret-name
      #       key: secret-key
...
  secret:
    dbSecret:
      enabled: false # Enable to apply database-secret.yaml which allows you to specify db credentials
    # -- Use an existing secret for the database variables: DB_NAME, DB_HOST, DB_USER, DB_PASSWORD
    existingDbSecret:
      # -- Enabled the use of the already created secret
      enabled: true

• the ability to use a preexisting secret for the license file:

  license:
    name: nexus-repo-license.lic
    # -- Use an already created secret with the license contents in base64, it takes the name from the above field -> name: nexus-repo-license.lic and the mount path from below: licenseSecret.mountPath
    existingSecret: false

Also there are a couple of formatting fixes mostly done automatically by the IDE.

This has been tested in my environment and works as expected, also I have tested that it does not impact existing functionality.