adds replication_spec attribute to cluster resource

[tamarindd]

Adds a replication_spec attribute to the mongodbatlas_cluster resource.

Example:

  replication_spec = [{
      region          = "US_WEST_1"
      priority        = 5
      electable_nodes = 2
      read_only_nodes = 0
  }]

The build won't pass until this is merged: https://github.com/akshaykarle/go-mongodbatlas/pull/2

[akshaykarle]

Merged in https://github.com/akshaykarle/go-mongodbatlas/pull/2. Let me know once you update the vendor and this should start to run with the go-mongodbatlas client updated.

[akshaykarle]

Thanks for the PR :)