nmcli/connections: use the current value in default in ask_option()

For new connections, this ensures the value in square brackets on
interactive add are always correct.

Apart from that, this allows us to initialize some non-default values
before asking (such as making up an interface name for some software
devices), and inform the user about what we picked:

  Interface name [nm-bridge]:
56 jobs for lr/ask-mode in 41 minutes and 24 seconds (queued for 17 seconds)