Glance image PVC contains requires images that should be careful handled
during changing glance storage when migrate out of PVC mode for glance
storage.
Which in migrate/upgrade path, should be be correctly moved and deleted
the PVC after.
On the other hand, it's also possible to accidentally changes storage
mode out of `pvc` and lose the `glance-images` PVC which is unbearable
mistake.
Once storage mode set to `pvc, we should allow that PVC to be able to
stay and ready for reuse again until it's mannually deleted.
This add flag `keep_pvc` (default to true).
Set it to true to set helm/resource-policy to keep for glance-images.
Set it to false to allow helm delete glance-images PVC when request.
Change-Id: I9d0e2a49aabf81eb2d4e00ad2a9d42125261489e
f074a65ba1