sg start ignores overwrite flag

Created by: asdine

The overwrite and config flags are ignored:

$ sg -overwrite ./foo/a.yaml  start my-set
ERROR: commandset "my-set" not found :(

The issue is caused by the constructStartCmdLongHelp function, which parses the config before the flags are parsed by ffcli. It uses the default value for the -overwrite flag (sg.config.overwrite.yaml) then passes it to the parseConf function which then fails silently if the file doesn't exist, and sets the globalConf variable with the parsed config. Subsequent calls to parseConf check if globalConf is set, and skip the loading of the configuration.