ini "exclude" config is ignored

Describe the bug
exclude configuration inside .bandit ini file is always ignored due to default cli argument.

To Reproduce
Steps to reproduce the behavior:

1. Create . bandit file with the following.

[bandit]
targets: .
exclude: ./tests

2. Run CLI with bandit --ini .bandit -r in a directory with .bandit file.
3. You should now see that Bandit is prefer CLI argument for exclude over ini.

[main]	INFO	Using command line arg for excluded paths
[main]	INFO	Using ini file for selected targets

Expected behavior
Bandit should use .bandit configuration as there's no CLI argument supplied then append default value to it as specified in argparse.

  -x EXCLUDED_PATHS, --exclude EXCLUDED_PATHS
                        comma-separated list of paths (glob patterns supported) to exclude
                        from scan (note that these are in addition to the excluded paths
                        provided in the config file) (default:
                        .svn,CVS,.bzr,.hg,.git,__pycache__,.tox,.eggs,*.egg)

Bandit version

bandit 1.7.0
  python version = 3.8.0 (default, Sep  3 2020, 18:08:13) [Clang 11.0.3 (clang-1103.0.32.62)]

Additional context
I believe this happens due to the default value for -x given to argparse. This means the ini configuration is always ignored as the code take default value as supplied CLI.

Workaround
Supplied exclude via CLI when running Bandit regardless of your ini. bandit -x ./tests --ini .bandit -r .

Start in bandit/cli/main.py around the argparse definition for -x and trace how its default is distinguished from an explicitly supplied CLI value. Reproduce with bandit --ini .bandit -r, then verify that the ini exclude is honored while the documented default paths remain included and an explicit -x still takes precedence.