Add check and reporting to apache_mod_cgi_bash_env

And some additional improvements.

## Verification steps:

- [ ] Run the check command on ```auxiliary/scanner/http/apache_mod_cgi_bash_env.rb``` against a vulnerable setup, make sure it flags the host as vulnerable.
- [ ] Run ```auxiliary/scanner/http/apache_mod_cgi_bash_env.rb``` agains a vulnerable setup. Make sure the command executes.
Since you are enforcing the check method, it makes more sense to call ```report_vuln``` in the check method, that way you don't have to check the marker twice.