Generate unique subdirs for built elements

Based on issue 89 (#89)
Ensuring that elements are staged into unique subdirs while building.
This patch supports that by doing the following:

* Modify project config to add 2 new variables ("project-name" and "element-name")
* Changed the default install-root from "/buildstream/install" to "/buildstream-install"
* Update the tests to accommodate these changes
* Update the expected cache keys in the tests

This fixes #89