fix: Print the long options of the required flags missing

The error message collected by 'checkRequiredFlags()' now sets the name of the flags
to the long option given in the 'Name: <somecommand>' field.

This change is introduced to fix up an error where printing the error in the
case of a missing required flag looks like:

```
Required flags " n,  t" not set
```

Whilst the expected behaviour is:

```
Required flags "<long-name1>, <long-name2>" not set
```

Signed-off-by: Ole Petter <ole.orhagen@northern.tech>

context.go

@@ -324,11 +324,12 @@ func checkRequiredFlags(flags []Flag, context *Context) requiredFlagsErr {
 			var flagPresent bool
 			var flagName string
 			for _, key := range strings.Split(f.GetName(), ",") {
+				key = strings.TrimSpace(key)
 				if len(key) > 1 {
 					flagName = key
 				}
 
-				if context.IsSet(strings.TrimSpace(key)) {
+				if context.IsSet(key) {
 					flagPresent = true
 				}
 			}