nginx-plus: handle POST calls returning 404 till state is updated, use common transport

[vishnunaini]

1. Use common transport for api calls
2. Initialiser server ID=0 for new upstream that doesn't exist as UpdateHTTPServers relies on GetHTTPServers to return valid data to determine diff to update
3. Wait for newly initialised upstream to return valid data

[vishnunaini]

This is a workaround till nginx client is updated to v2.2+ as the workaround is not necessary with this fix although the 404 behaviour isn't fixed nginx/nginx-plus-go-client#422