remove MixedInstancesPolicy from nodeup MIMEBOUNDARY

[Deshke]

1. Describe IN DETAIL the feature/behavior/change you would like to see.

with the merge from #6732, the nodeup script

kops/pkg/model/resources/nodeup.go

Line 192 in 70d7ac7

if len(ig.Spec.AdditionalUserData) > 0 || ig.Spec.MixedInstancesPolicy != nil {

changed to include the MixedInstancesPolicy, but MixedInstancesPolicy does not change anything on the user-data and multipart content breaks coreOS-cloudinit coreos/bugs#2588

Content-Type: multipart/mixed; should be used if there are multiple scripts but there is only one being provided and ig.Spec.MixedInstancesPolicy does not change that.

2. Feel free to provide a design supporting your feature request.

• Content-Type: multipart/mixed; should only be set if multiple scripts are provided