We can have an uppercase member interface in the bridge config
```
bridge br1 {
enable-vlan
member {
interface Vxlan1 {
}
}
}
```
But an upper case VXLAN interface can not be created.
During debugging of this I also noticed that there are multiple different calls to confi.exists() in vyos.configdict - some use string, some use list as argument.
This has been streamlined to only make use of lists.