Use pip provider to install using pip

These execs were introduced in 6a484ff89 to avoid redownloading
eggs. The pip provider has since changed to use `pip freeze` to avoid
downloading eggs for modules which have already been installed
locally, so these exec resources can revert to package { provider =>
pip} resources.
make librarian-puppet happier
use ensure_packages rather than Package[] resources
make pip provider depend on python-pip
Merge branch 'make_pip_stuff_nicer' of github.com:philandstuff/garethr-graphite into philandstuff-make_pip_stuff_nicer

Conflicts:
	Modulefile
	manifests/install.pp
fix tests due to move to package from exec