deploy: Add --no-merge

All of the underlying libostree APIs have supported passing `NULL`
for a merge deployment for...a long time.  But we never plumbed
it up into the CLI.

Add a `--no-merge` option to aid people who want to do a "factory reset":
https://github.com/ostreedev/ostree/issues/1793

.cci.jenkinsfile

@@ -74,6 +74,7 @@ parallel fcos: {
         mkdir -p overrides/rootfs
         mv insttree/* overrides/rootfs/
         rmdir insttree
+        coreos-assembler fetch
         coreos-assembler build
       """)
     }