Fix env.example Default Values (#3827)

* Set Azure prefix and USER as optional at env.example

Follow up commit to use an Azure unique prefix for the Azure resources that ARO-RP is using instead of always fetching the USER. When AZURE_PREFIX env var is not set, then use the USER env var

* Use westeurope as default location

Don't override existed LOCATION env var when it is already set, and 'westeurope' as the default value

env.example

@@ -1,12 +1,12 @@
 # use unique prefix for Azure resources when it is set, otherwise use your user's name
 export AZURE_PREFIX="${AZURE_PREFIX:-$USER}"
-export LOCATION=westeurope
+export LOCATION="${LOCATION:-westeurope}"
 export NO_CACHE=false
 export AZURE_EXTENSION_DEV_SOURCES="$(pwd)/python"
 
-export CLUSTER_RESOURCEGROUP="${USER}-v4-$LOCATION"
-export CLUSTER_NAME="${USER}-aro-cluster"
-export CLUSTER_VNET="${USER}-aro-vnet"
+export CLUSTER_RESOURCEGROUP="${AZURE_PREFIX}-v4-$LOCATION"
+export CLUSTER_NAME="${AZURE_PREFIX}-aro-cluster"
+export CLUSTER_VNET="${AZURE_PREFIX}-aro-vnet"
 export ARO_IMAGE=arointsvc.azurecr.io/aro:latest 
 
 . secrets/env