Network tests

All changes are to test code, so this should have no end-user impact.  It does, of course, improve the tests :+1: 

This adds integration tests for KNIFE-267 (Rackspace server create with networks), which was coded before KNIFE-289 (Add Integration Tests) was merged.

There are also some minor test improvements and refactorings, like splitting the tests from just "integration" to "server", "network", "image" and "flavor" tests, adding code coverage via SimpleCov, reducing the number of environment variables required to run live integration tests, and running server creation tests during live testing (though they are still skipped while using cassettes).

In order to run integration tests, you just need to set two environment variables:
OS_USERNAME=<your rackspace username>
OS_PASSWORD=<your rackspace api key>

*rake integration:test* runs tests with previously recorded cassettes
*rake integration:live* runs live tests against your rackspace account
*rake* runs both - cassettes then live