BUG/MEDIUM: config: add default location of path to the dataplaneapi configuration file

default location was not set, and therefore configuration file was not used.
this results in loosing service discovery data if that file is not set when
dataplaneapi is restarted

Author: oktalz

configuration/configuration.go

@@ -48,7 +48,7 @@ type HAProxyConfiguration struct {
 	BackupsNumber        int    `short:"n" long:"backups-number" description:"Number of backup configuration files you want to keep, stored in the config dir with version number suffix" default:"0"`
 	MasterRuntime        string `short:"m" long:"master-runtime" description:"Path to the master Runtime API socket"`
 	ShowSystemInfo       bool   `short:"i" long:"show-system-info" description:"Show system info on info endpoint"`
-	DataplaneConfig      string `short:"f" description:"Path to the dataplane configuration file" default:"" yaml:"-"`
+	DataplaneConfig      string `short:"f" description:"Path to the dataplane configuration file" default:"dataplaneapi.yaml" yaml:"-"`
 	UserListFile         string `long:"userlist-file" description:"Path to the dataplaneapi userlist file. By default userlist is read from HAProxy conf. When specified userlist would be read from this file"`
 	NodeIDFile           string `long:"fid" description:"Path to file that will dataplaneapi use to write its id (not a pid) that was given to him after joining a cluster"`
 	MapsDir              string `short:"p" long:"maps-dir" description:"Path to directory of map files managed by dataplane" default:"/etc/haproxy/maps"`