Make Berksfile path configurable

Berkshelf provides the ability to use custom `Berksfile` paths with its `berks` command using the `--berksfile` flag. This pull request adds support for custom `Berksfile` paths to ChefSpec.

The use case I have involves two separate `Berksfile`s: one for installing cookbook dependencies on OpsWorks, and another that includes OpsWorks cookbooks for testing locally and in CI builds. Including OpsWorks cookbooks in the `Berksfile` used by OpsWorks could cause conflicts.

I tried to write tests, but it looked like `ChefSpec::Berkshelf` was untested and difficult to add tests for.