fix_rules.py: Set subcommand required

[ggbecker]

Description:

• fix_rules.py: Set subcommand required so it prints a nice message when there is no subcommand present in the command line.

Rationale:

• Do not traceback when there is no subcommand, for example:

python3 utils/fix_rules.py
usage: fix_rules.py [-h] [-y] [-d] [-j JSON] [-r ROOT] [--product PRODUCT]
                    {empty_identifiers,empty_references,prefixed_identifiers,invalid_identifiers,int_identifiers,int_references,duplicate_subkeys,sort_subkeys,sort_prodtypes,add-cce} ...
fix_rules.py: error: the following arguments are required: {empty_identifiers,empty_references,prefixed_identifiers,invalid_identifiers,int_identifiers,int_references,duplicate_subkeys,sort_subkeys,sort_prodtypes,add-cce}

Before:

python3 utils/fix_rules.py
Traceback (most recent call last):
  File "/home/ggasparb/workspace/github/content/utils/fix_rules.py", line 808, in <module>
    __main__()
  File "/home/ggasparb/workspace/github/content/utils/fix_rules.py", line 804, in __main__
    args.func(args, subst_dict)
AttributeError: 'Namespace' object has no attribute 'func'