Add NM_SETTING_SECRET_FLAG_NOT_SAVED to challenge response. This solves
2 problems:
1. The response was saved to the NM profile and some agents tried to
   reuse it, which is wrong because it changes each time. This flag
   prevents that.
2. Nmcli and other clients using nm-secret-agent-simple (if there are
   any) ignore any request for secrets that don't have either
   NM_SETTING_SECRET_FLAG_NOT_SAVED nor
   NM_SETTING_SECRET_FLAG_AGENT_OWNED.
   For challenge-response, it mustn't be agent owned because, as
   already said, it changes each time so agents with storing
   capabilities neither have to store it.

The flag is added both when importing an .ovpn file and when using the
editor.