Use a separate field for the "sync images with tag matching regex" feature

Fields that magically change their behavior depending on type of the value
are too much hassle for no benefit.

For now, this just copies&pastes the full loop in imagesToCopyFromRegistry
to create another loop handling the new ImagesByTagRegex field. Simplifications
to reduce duplication will follow.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

integration/sync_test.go

@@ -263,7 +263,7 @@ func (s *SyncSuite) TestYamlRegex2Dir(c *check.C) {
 
 	yamlConfig := `
 docker.io:
-  images:
+  images-by-tag-regex:
     nginx: ^1\.13\.[12]-alpine-perl$  # regex string test
 `
 	// the       ↑    regex strings always matches only 2 images