deploy: Add an assertion that deployment directory exists
deploy: Add an assertion that deployment directory exists

I may be seeing something going wrong in rpm-ostree, just adding this
assertion for my own sanity.

leandrosansilva/ostree,gatispaeglis/ostree,leandrosansilva/ostree,gatispaeglis/ostree,gatispaeglis/ostree,resin-io/ostree,leandrosansilva/ostree,leandrosansilva/ostree,sujitfulse/ostree,leandrosansilva/ostree,gatispaeglis/ostree,sujitfulse/ostree,gatispaeglis/ostree,resin-io/ostree,sujitfulse/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,resin-io/ostree,sujitfulse/ostree,gatispaeglis/ostree,GNOME/ostree,resin-io/ostree,resin-io/ostree,sujitfulse/ostree