Adds `nopuppetrollback` capistrano task.

Adds the `nopuppetrollback` capistrano task so that Puppet failures
won't cause a rollback. It passes the `--ignore` option to
shadow_puppet.
Merge pull request #180 from bryantraywick/add-nopuppetrollback-cap-task-so-that-puppet-failures-wont-cause-a-rollback

Adds `nopuppetrollback` capistrano task.