Modify tenantConfig.json examples to use apikey instead of username/password

[jfmartinez]

A lot of our samples contain username/password which still work by specifying apikey as the username and the as the password, but it's not straightforward. We should replace the samples as such:

{
   ...
   "credentials": {
           "username": "apikey",
           "password": "<APIKEY>"
     }
}

To:

{
   ...
   "credentials": {
           "apikey": "<APIKEY>"
     }
}