Make 'SkipContents' configurable in API. #345

Also add global configuration to disable 'skipContents' by
default for all new published repos/snapshots.

utils/config_test.go

@@ -62,6 +62,7 @@ func (s *ConfigSuite) TestSaveConfig(c *C) {
 		"  \"downloadSourcePackages\": false,\n"+
 		"  \"ppaDistributorID\": \"\",\n"+
 		"  \"ppaCodename\": \"\",\n"+
+		"  \"skipContentsPublishing\": false,\n"+
 		"  \"S3PublishEndpoints\": {\n"+
 		"    \"test\": {\n"+
 		"      \"region\": \"us-east-1\",\n"+