[sonic-cfggen] Load JSON files before minigraph file

[jleveque]

- What I did

If sonic-cfggen is passed the -m argument (to load the minigraph file) along with one or more -j <json_file> arguments, load the JSON files before loading the minigraph file.

This ensures that the init_cfg.json file is loaded before the minigraph, therefore the minigraph can override any default configuration options specified in init_cfg.json. Currently, the behavior is reversed.

Note: This is not an issue if loading loading multiple JSON files, because sonic-cfggen loads them in the left-to-right order they were specified on the command line, therefore providing flexibility for loading JSON files in a specific order. As long as init_cfg.json is specified before config_db.json, the values specified in config_db.json will take precedence.

- How I did it

- How to verify it

1. Create a minigraph.xml file and an init_cfg.json file which have different values for the same key(s).
2. Run sonic-cfggen -H -m -j /etc/sonic/init_cfg.json --write-to-db
3. Run config save -y
4. Inspect the resulting config_db.json file to ensure that the value from the minigraph.xml file is the value present, not the value from init_cfg.json.