[rabbitmq_credentials] remove unnecessary reference to undefined variable "key"
[rabbitmq_credentials] remove unnecessary reference to undefined variable "key"

Seems that #466 contains a regression causing test-kitchen runs to fail due to
undefined variable "key" being passed when invoking rabbitmq_credentials
definition. I believe removing this undefined variable from the arguments will
fix the problem and the code will otherwise function as expected.

Needs tests :)

sensu/sensu-chef,klamontagne/sensu-chef,cwjohnston/sensu-chef,sensu/sensu-chef,klamontagne/sensu-chef,cwjohnston/sensu-chef