Handle secretKeyRef in ContainerBuilder

[tommaso-borgato]

This MR add the option to use secretKeyRef, besides configMapKeyRef, in DeploymentConfig generated by ContainerBuilder, e.g.:

        - name: KC_DB_USERNAME
          valueFrom:
            secretKeyRef:
              name: postgresql
              key: database-user
        - name: KC_DB_PASSWORD
          valueFrom:
            secretKeyRef:
              name: postgresql
              key: database-password

Please make sure your PR meets the following requirements:

• [x] Pull Request contains a description of the changes
• [x] Pull Request does not include fixes for multiple issues/topics
• [x] Code is formatted, imports ordered, code compiles and tests are passing
• [x] Code is self-descriptive and/or documented

[mnovak1]

LGTM, merging. Thanks for the fix.