COOK-2453: adding directory parameter to control gunicorn cwd

In some scenarios it is important that gunicorn be executed
from a path other than the current release directory. This
change adds a 'directory' attribute to the gunicorn submodule.
When set, the value of this attribute is used to configure
gunicorn's working directory.
Merge pull request #20 from needle-cookbooks/COOK-2453

Add a directory parameter to control gunicorn working directory