Raise ConfigFilesNotFoundError if CONF.api_paste_config not found

Otherwise it shows an incomprehensible message
"'NoneType' object has no attribute 'startswith'."

Change-Id: I5d87c4ceac69b26730d63727a2ec3701ccc9cecb
Closes-bug: 1236182
Merge "Raise ConfigFilesNotFoundError if CONF.api_paste_config not found"