Accept "overlay2.imagestore" as a synonym option
Accept "overlay2.imagestore" as a synonym option

Accept "overlay2.imagestore" as a synonym for the "overlay.imagestore"
option, and be a bit more consistent about using the driver name we were
invoked by in error messages.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

rhatdan/storage,containers/storage,rhatdan/storage,containers/storage,rhatdan/storage,containers/storage