feat(cli): allow specifying name of provisioner daemon (#11077)

- Adds a --name argument to provisionerd start
- Plumbs through name to integrated and external provisioners
- Defaults to hostname if not specified for external, hostname-N for integrated
- Adds cliutil.Hostname

cli/cliutil/hostname.go

@@ -0,0 +1,40 @@
+package cliutil
+
+import (
+	"os"
+	"strings"
+	"sync"
+)
+
+var (
+	hostname     string
+	hostnameOnce sync.Once
+)
+
+// Hostname returns the hostname of the machine, lowercased,
+// with any trailing domain suffix stripped.
+// It is cached after the first call.
+// If the hostname cannot be determined, for any reason,
+// localhost will be returned instead.
+func Hostname() string {
+	hostnameOnce.Do(func() { hostname = getHostname() })
+	return hostname
+}
+
+func getHostname() string {
+	h, err := os.Hostname()
+	if err != nil {
+		// Something must be very wrong if this fails.
+		// We'll just return localhost and hope for the best.
+		return "localhost"
+	}
+
+	// On some platforms, the hostname can be an FQDN. We only want the hostname.
+	if idx := strings.Index(h, "."); idx != -1 {
+		h = h[:idx]
+	}
+
+	// For the sake of consistency, we also want to lowercase the hostname.
+	// Per RFC 4343, DNS lookups must be case-insensitive.
+	return strings.ToLower(h)
+}