PDK .sync.yml allow for removing template values

This implements a new feature for PDK that allows users to remove default template configuration values by using the "knockout_prefix" of `---` in their `.sync.yml` file. An example of doing this would be: In default configs for pdk-templates, we have this defined for `.travis.yml`: ``` .travis.yml:   bundler_args: --without system_tests ``` And I want to remove that value and have bundler install everything. I would put this in my `.sync.yml`. ``` --- .travis.yml:   bundle_args: -----without system_tests ``` By prepending the `knockout_prefix` of `---` to the value you want to remove, PDK will remove that value when we render the `.travis.yml` file when PDK update or convert is run. ``` /* Report generated by PDK at 2018-06-14 14:40:48 -0700 */ --- .travis.yml 2018-06-14 10:50:49.429639601 -0700 +++ .travis.yml.pdknew 2018-06-14 14:40:48.632016000 -0700 @@ -11,7 +11,7 @@    - bundle -v  script:    - 'bundle exec rake $CHECK' -bundler_args: --without system_tests +bundler_args:  rvm:    - 2.4.1  env: ```