manifest: Add container-cmd

Builds on coreos/rpm-ostree#3402

Relates to coreos/coreos-assembler#2685

Basically, source of truth for the CMD moves from being hardcoded
in cosa hackiliy to being part of the ostree commit, which means
it survives a full round trip of
ostree → container → ostree → container

manifest.yaml

@@ -4,6 +4,9 @@ variables:
 
 releasever: 35
 
+container-cmd:
+  - /usr/bin/bash
+
 repos:
   # These repos are there to make it easier to add new packages to the OS and to
   # use `cosa fetch --update-lockfile`; but note that all package versions are