Treat user defined metadata case-insensitive

Convert all user defined metadata names to uppercase before locally
saving them to the metadata dictionary. The metadata names are case
insensitive and by converting them to uppercase we can ensure that at
any moment only one instance of them is present in the dictionary.

This fixes #39

image_creator/main.py

@@ -182,7 +182,7 @@ def parse_options(input_args):
         except ValueError:
             raise FatalError("Metadata option: `%s' is not in KEY=VALUE "
                              "format." % m)
-        meta[key] = value
+        meta[key.upper()] = value
     options.metadata = meta
 
     sysprep_params = {}