Some fixed for new LABEL stuff

- command.Commands was missing "Label"
- used the correct error string in dispatcher when LABEL has no args, otherwise
  the test TestBuildMissingArgs will not work
- removed the premature error msg in line_parser that was blocking the
  label() func in dispatcher from showing the err msg in previous bullet
- since LABEL uses the env parser it needs to be added to the replaceEnvAllowed
  list so that proper quote processing will be done.  Especially once
  PR #10431 is merged.

Signed-off-by: Doug Davis <dug@us.ibm.com>

builder/parser/line_parsers.go

@@ -137,7 +137,7 @@ func parseNameVal(rest string, key string) (*Node, map[string]bool, error) {
 	}
 
 	if len(words) == 0 {
-		return nil, nil, fmt.Errorf(key + " requires at least one argument")
+		return nil, nil, nil
 	}
 
 	// Old format (KEY name value)