allow templating {stream} in image.yaml files

This will allow us to provide the following, essentially allowing
us to have the same image.yaml on all branches, but allowing for the
stream name to get substituted in dynamically.

```
container-imgref: "ostree-remote-registry:fedora:quay.io/fedora/fedora-coreos:{stream}"
```

Not having something like this ended up with us mismanaging the
image.yaml during the promotion of `next-devel` to `next` in
coreos/fedora-coreos-tracker#1913

This was proposed as a better alternative to modifying promote-config
to ignore `image.yaml`: coreos/fedora-coreos-releng-automation#202

Author: dustymabe

src/cmdlib.sh

@@ -206,7 +206,7 @@ prepare_build() {
     fi
 
     export image_json="${tmp_builddir}/image.json"
-    write_image_json "${image}" "${image_json}"
+    write_image_json "${image}" "${image_json}" "${manifest}"
     # These need to be absolute paths right now for rpm-ostree
     composejson="$(readlink -f "${workdir}"/tmp/compose.json)"
     export composejson
@@ -1075,11 +1075,12 @@ print('Build ${buildid} was inserted ${arch:+for $arch}')")
 write_image_json() {
     local srcfile=$1; shift
     local outfile=$1; shift
+    local ostree_manifest=$1; shift
     (python3 -c "
 import sys
 sys.path.insert(0, '${DIR}')
 from cosalib import cmdlib
-cmdlib.write_image_json('${srcfile}', '${outfile}')")
+cmdlib.write_image_json('${srcfile}', '${outfile}', ostree_manifest='${ostree_manifest}')")
 }
 
 # API to prepare image builds.

src/cosalib/cmdlib.py

@@ -467,15 +467,16 @@ def cmdlib_sh(script):
     '''])
 
 
-def generate_image_json(srcfile):
+def generate_image_json(srcfile, ostree_manifest):
+    manifest_vars = yaml.safe_load(open(ostree_manifest))['variables']
     r = yaml.safe_load(open("/usr/lib/coreos-assembler/image-default.yaml"))
-    for k, v in flatten_image_yaml(srcfile).items():
+    for k, v in flatten_image_yaml(srcfile, format_args=manifest_vars).items():
         r[k] = v
     return r
 
 
-def write_image_json(srcfile, outfile):
-    r = generate_image_json(srcfile)
+def write_image_json(srcfile, outfile, ostree_manifest):
+    r = generate_image_json(srcfile, ostree_manifest)
     with open(outfile, 'w') as f:
         json.dump(r, f, sort_keys=True)
 
@@ -490,12 +491,13 @@ def merge_lists(x, y, k):
     x[k].extend([i for i in y[k] if i not in x[k]])
 
 
-def flatten_image_yaml(srcfile, base=None):
+def flatten_image_yaml(srcfile, base=None, format_args={}):
     if base is None:
         base = {}
 
     with open(srcfile) as f:
-        srcyaml = yaml.safe_load(f)
+        contents = f.read()
+        srcyaml = yaml.safe_load(contents.format(**format_args))
 
     # first, special-case list values
     merge_lists(base, srcyaml, 'extra-kargs')
@@ -508,7 +510,7 @@ def flatten_image_yaml(srcfile, base=None):
 
     fn = os.path.join(os.path.dirname(srcfile), srcyaml['include'])
     del base['include']
-    return flatten_image_yaml(fn, base)
+    return flatten_image_yaml(fn, base=base, format_args=format_args)
 
 
 def ensure_glob(pathname, **kwargs):