promptChoiceOnce map path prompt choices [default]

promptChoiceOnce returns the value of map at path if it exists and is a string, otherwise it prompts the user for one of choices with prompt and an optional default using promptChoice.

!!! example

```
{{- $choices := list "desktop" "laptop" "server" "termux" -}}
{{- $hosttype := promptChoiceOnce . "hosttype" "What type of host are you on" $choices -}}
[data]
    hosttype = {{- $hosttype | quote -}}
```