nomad agent vault block config is not validated

[shoenig]

While trying to investigate #13062 I had this Nomad agent vault config:

vault {                                                                                                                                                                   
  enabled = true                                                                                                                                                          
  address = "http://127.0.0.1:8200"                                                                                                                                       
  token = "<token>"                                                                                                                                                                                                                                                               
  allows_unauthenticated = false                                                                                                                                          
} 

This is not valid as allows_unauthenticated should have been allow_unauthenticated, which is particularly bad since the config silently allows using vault unauthenticated.