"delete" command fails if credentials not specified

[movitto]

Attempting to run 'imagefactory delete' against a base image is failing due to missing credentials. Specifically /usr/bin/imagefactory attempts to invoke 'read().strip()' on app_config.get('credentials') even if the return value is None:

https://github.com/redhat-imaging/imagefactory/blob/master/imagefactory#L275

This line should be changed to use the _get_credentials helper in the same manner as the 'provider_image' command