Add flag to disable/enable support for legacy pool paths

Legacy pool paths are enabled by default, but for new aptly installations
(when aptly config is first generated), it would be disabled explicitly.

utils/config.go

@@ -20,6 +20,7 @@ type ConfigStructure struct {
 	GpgDisableSign         bool                             `json:"gpgDisableSign"`
 	GpgDisableVerify       bool                             `json:"gpgDisableVerify"`
 	DownloadSourcePackages bool                             `json:"downloadSourcePackages"`
+	SkipLegacyPool         bool                             `json:"skipLegacyPool"`
 	PpaDistributorID       string                           `json:"ppaDistributorID"`
 	PpaCodename            string                           `json:"ppaCodename"`
 	SkipContentsPublishing bool                             `json:"skipContentsPublishing"`
@@ -81,6 +82,7 @@ var Config = ConfigStructure{
 	GpgDisableSign:         false,
 	GpgDisableVerify:       false,
 	DownloadSourcePackages: false,
+	SkipLegacyPool:         false,
 	PpaDistributorID:       "ubuntu",
 	PpaCodename:            "",
 	FileSystemPublishRoots: map[string]FileSystemPublishRoot{},

utils/config_test.go

@@ -65,6 +65,7 @@ func (s *ConfigSuite) TestSaveConfig(c *C) {
 		"  \"gpgDisableSign\": false,\n"+
 		"  \"gpgDisableVerify\": false,\n"+
 		"  \"downloadSourcePackages\": false,\n"+
+		"  \"skipLegacyPool\": false,\n"+
 		"  \"ppaDistributorID\": \"\",\n"+
 		"  \"ppaCodename\": \"\",\n"+
 		"  \"skipContentsPublishing\": false,\n"+