Made pip install step configurable.

The pip install step gets its own state in venv.sls, and various venv requirements key off of it rather than the creation of the venv itself. This allow for customization of the pip install command for options such as --find_link, --no_index, --upgrade, and so on.

Existing projects that wish to migrate to this way of doing things can use this glorious command to convert their require clauses over (from /conf/salt/project/):

find . -type f -print0 | xargs -0 sed -i 's/virtualenv: venv/pip: pip_requirements/'