kpm diagnostic tool behaves inconsistently when the api credentials are specified in a config file as well as on the command line

[reshmabidikar]

Steps to reproduce:

1. Create a config.yml file as follows:

killbill:
  api_key: bob
  api_secret: lazar
  port: 8080
  host: localhost
  user: admin
  password: password
  plugins_dir: /var/tmp/bundles

2. Run the kpm diagnostic command as follows:

kpm diagnostic --account-export=<account id from bob/lazar tenant> --config_file=config.yml --killbill-api-credentials=t1 t1 

3. In this case, the diagnostic file contains tenant information from the t1/t1 tenant and account information from the bob/lazar tenant.

Expected Behavior: We should disallow specifying both a config file and command line options to prevent such inconsistent behavior.