Add back image_pull_secret field for backwards compatibility

[rooftopcellist]

#860 introduced a new field (image_pull_secrets) which allows users to specify multiple pull secrets if needed for app and database container images. The old field on the CRD (image_pull_secret) was removed at that time.

This patch adds that back for backwards compatibility. Without this change, users who specify it the old way will be met with a validator error when applying the AWX CR.

The new syntax that should be used is:

---
apiVersion: awx.ansible.com/v1beta1
kind: AWX
metadata:
  name: awx
spec:
  image_pull_secrets:
    - pull-secret