generate-release-meta: Support customizing the base URL

Needed for RHCOS.

src/cmd-generate-release-meta

@@ -25,7 +25,7 @@ def ensure_dup(inp, out, inp_key, out_key):
 
 
 def url_builder(stream, version, arch, path):
-    return f"https://builds.coreos.fedoraproject.org/prod/streams/{stream}/builds/{version}/{arch}/{path}"
+    return f"{args.stream_baseurl}/{stream}/builds/{version}/{arch}/{path}"
 
 
 def get_extension(path, modifier, arch):
@@ -35,6 +35,7 @@ def get_extension(path, modifier, arch):
 parser = ArgumentParser()
 parser.add_argument("--workdir", help="cosa workdir")
 parser.add_argument("--build-id", help="build id")
+parser.add_argument("--stream-baseurl", help="Prefix URL for stream content", default="https://builds.coreos.fedoraproject.org/prod/streams")
 parser.add_argument("--output", help="Output to file; default is build directory")
 parser.add_argument("url", metavar='URL', help="URL to a coreos-assembler meta.json", default=[], nargs='*')
 args = parser.parse_args()