Added -version flag to watchdog

This changes introduces a new flag -version to watchdog which will
display version and SHA of last git commit.

Version and SHA are injected at build time and passed as a
build-args for Dockerfile.

Fixes: #632

Signed-off-by: Vivek Singh <vivekkmr45@yahoo.in>

watchdog/version.go

@@ -0,0 +1,18 @@
+package main
+
+var (
+	//Version release version of the watchdog
+	Version string
+	//GitCommit SHA of the last git commit
+	GitCommit string
+	//DevVerison string for the development version
+	DevVerison = "dev"
+)
+
+//BuildVersion returns current version of watchdog
+func BuildVersion() string {
+	if len(Version) == 0 {
+		return DevVerison
+	}
+	return Version
+}