[FAB-8473] Make configtxlator truncate output file

The new configtxlator CLI mode allows specifying an output file.  If the
file already exists, the output is appended.  This is fairly unintuitive
behavior or a CLI and it should truncate the file instead.  This CR
simply adds the truncate flag to the file opens in the flag options.

Change-Id: I5ea14170da1dc93f4f5c5b9aa555080311f71e9e
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>

common/tools/configtxlator/main.go

@@ -48,18 +48,18 @@ var (
 	protoEncode       = app.Command("proto_encode", "Converts a JSON document to protobuf.")
 	protoEncodeType   = protoEncode.Flag("type", "The type of protobuf structure to encode to.  For example, 'common.Config'.").Required().String()
 	protoEncodeSource = protoEncode.Flag("input", "A file containing the JSON document.").Default(os.Stdin.Name()).File()
-	protoEncodeDest   = protoEncode.Flag("output", "A file to write the output to.").Default(os.Stdout.Name()).OpenFile(os.O_RDWR|os.O_CREATE, 0600)
+	protoEncodeDest   = protoEncode.Flag("output", "A file to write the output to.").Default(os.Stdout.Name()).OpenFile(os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
 
 	protoDecode       = app.Command("proto_decode", "Converts a proto message to JSON.")
 	protoDecodeType   = protoDecode.Flag("type", "The type of protobuf structure to decode from.  For example, 'common.Config'.").Required().String()
 	protoDecodeSource = protoDecode.Flag("input", "A file containing the proto message.").Default(os.Stdin.Name()).File()
-	protoDecodeDest   = protoDecode.Flag("output", "A file to write the JSON document to.").Default(os.Stdout.Name()).OpenFile(os.O_RDWR|os.O_CREATE, 0600)
+	protoDecodeDest   = protoDecode.Flag("output", "A file to write the JSON document to.").Default(os.Stdout.Name()).OpenFile(os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
 
 	computeUpdate          = app.Command("compute_update", "Takes two marshaled common.Config messages and computes the config update which transitions between the two.")
 	computeUpdateOriginal  = computeUpdate.Flag("original", "The original config message.").File()
 	computeUpdateUpdated   = computeUpdate.Flag("updated", "The updated config message.").File()
 	computeUpdateChannelID = computeUpdate.Flag("channel_id", "The name of the channel for this update.").Required().String()
-	computeUpdateDest      = computeUpdate.Flag("output", "A file to write the JSON document to.").Default(os.Stdout.Name()).OpenFile(os.O_RDWR|os.O_CREATE, 0600)
+	computeUpdateDest      = computeUpdate.Flag("output", "A file to write the JSON document to.").Default(os.Stdout.Name()).OpenFile(os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
 
 	version = app.Command("version", "Show version information")
 )