Run tox against installed version.

By default tox runs tests in source directory.
Don't reinstall flocker package in tox.

tox builds an sdist and installs it automatically. Installing the source
directory as well is redundant and muddles testing of installing from an sdist.
Add a comment about why we change directory.
Merge pull request #227 from ClusterHQ/tox-installed

Tox cleanups.