Add configurable repository clone directory

This changeset adds an additional attribute to the role for specifying a
directory for the Git clone. Underneath the covers, this maps to the
`dest` parameter of the Ansible Git module.

In this case, it defaults to:

  /opt/wordpress-{{ wordpress_site_name }}-{{ wordpress_site_tag }}
Merge pull request #6 from azavea/feature/repository-directory

Add configurable repository clone directory