feat(logs): 'uc machine logs' to view logs from systemd services on machines (#283)

* logging: client side

Create internal/logs and put abstracted way service and machine logging in there
This adds the bits to do machine logging of the services.

See #273 and discussion in #158

Signed-off-by: Miek Gieben <miek@miek.nl>

* go mod tidy

Signed-off-by: Miek Gieben <miek@miek.nl>

* fix typo

Signed-off-by: Miek Gieben <miek@miek.nl>

* Hook it up further

Signed-off-by: Miek Gieben <miek@miek.nl>

* Check unit validity server side

Signed-off-by: Miek Gieben <miek@miek.nl>

* Make cli-docs

Signed-off-by: Miek Gieben <miek@miek.nl>

* Add newline to logs

Signed-off-by: Miek Gieben <miek@miek.nl>

* docs

Signed-off-by: Miek Gieben <miek@miek.nl>

* Polish a bit

Signed-off-by: Miek Gieben <miek@miek.nl>

* comment fix

Signed-off-by: Miek Gieben <miek@miek.nl>

* There is no groupid

Signed-off-by: Miek Gieben <miek@miek.nl>

---------

Signed-off-by: Miek Gieben <miek@miek.nl>

internal/journal/journal.go

@@ -10,11 +10,31 @@ import (
 	"github.com/psviderski/uncloud/pkg/api"
 )
 
+const (
+	UnitUncloud   = "uncloud"
+	UnitDocker    = "docker"
+	UnitCorrosion = "uncloud-corrosion"
+)
+
+func ValidUnit(unit string) bool {
+	switch unit {
+	case UnitUncloud:
+	case UnitDocker:
+	case UnitCorrosion:
+	default:
+		return false
+	}
+	return true
+}
+
 const journalctl = "journalctl"
 
-var commandContext = exec.CommandContext // overidable for the test
+var commandContext = exec.CommandContext // allow override for test
 
 func logs(ctx context.Context, unit string, opts api.ServiceLogsOptions) (io.ReadCloser, error) {
+	if !ValidUnit(unit) {
+		return nil, fmt.Errorf("journal logs: invalid unit: %s", unit)
+	}
 	args := []string{"-u", unit, "--no-hostname"}
 	args = append(args, "-n")
 	if opts.Tail > -1 {