Split AWS and GCP cluster creation into separate controllers

The following discussion from !18307 (merged) should be addressed:

• @ayufan started a discussion: (+4 comments)

  Something is wrong with this #new. It really seems that this should be a separate controller for EKS, separate controller for GKE and User.

  Now, we mix a multiple responsibilities and add conditions to handle 3 different scenarios.

  The number of conditions in app/views/clusters/clusters/new.html.haml is also quite big, and this should rather be abstracted.