Is the belief that a consultant knows more than the team a form of cognitive bias?
Yes, this could be a cognitive bias. Quite clearly, a consultant is expected to be more knowledgeable about the specific topic they were hired to consult on.


  A consultant (from Latin: consultare "to deliberate") is a
  professional who provides expert advice.


Consultants are supposed to be experts in their field, and as a result they carry some authority. Therefore, authority bias comes to mind:


  Authority bias is the tendency to attribute greater accuracy to the
  opinion of an authority figure (unrelated to its content) and be more
  influenced by that opinion.


The bolded addendum in between parentheses is important. As long as based on the content the consultant brings to the table you respect their opinion, there is no cognitive bias (no irrationality); they are simply doing a good job.

But, not all 'consultants' are experts per se. In software development specifically, I have the impression 'consultancy' has almost become synonymous with 'temporary hire'. Newly grads often get hired straight out of school in consultancy firms. In such cases, there might be a cognitive bias at play, when it is not the contributed content which is evaluated (as you outline in your answer), but merely the title the consultant carries.