Extract a re-usable Python library for building integration tests

Currently, Tempest is only installable as a monolithic Python project that includes both the large number of integration, "scenario", and stress tests as well as all of the internal "plumbing" code that is used to, for instance, configure and discover the test environment.

What this means is that projects that are not an Integrated OpenStack project must currently copy/paste a bunch of code out of Tempest in order to construct "Tempest-compatible" functional integration tests for their project before it is integrated.

See here for a discussion on the -dev mailing list about this topic:

http://lists.openstack.org/pipermail/openstack-dev/2013-December/023087.html

This blueprint is for tracking progress towards splitting out the internal plumbing of Tempest from its massive collection of test cases, in order that other projects can simply do:

```python
import tempestlib

class MySuperFunctionalTest(tempestlib.TestWithScenarios):
  ...
```

for example, instead of copy/pasting a bunch of Tempest's code (and subsequently getting out-of-sync with Tempest's upstream enhancements.