cmd/silence: Can't set empty tags

[kylebrandt]

Due to:

https:/bosun-monitor/bosun/blob/master/cmd/silence/main.go#L40

if *flagTags == "" {
        *flagTags = "host=" + util.Hostname
    }

I fixed this problem while in annoate cli by doing the following:

    setHostname := false
    for _, arg := range os.Args {
        if arg == "-"+flagHostFlagKey {
            setHostname = true
        }
    }

Kind of a bit crude, but works. I should check in flag and see if there is a better way to do this.

Also, while in code I should make it so it doesn't take os/user since that uses Cgo. Did the following in annotate and it seems to work fine:

    un := *userFlag
    if un == "" {
        // Since os/user requires cgo
        un = os.Getenv(USER_ENV)
        sudo := os.Getenv("SUDO_USER")
        if sudo != "" {
            un = sudo
        }
    }