--use-leap-wheels sets --trusted-host (remove it when we have a proper
cert) and WHEELHOUSE to https://ftp.lizard.leap.se

Until we get ftp.lizard cname, use lizard as the wheels server.

- Related: #7339

Added requirements-latest to make sure we always use the
latest develop of all dependencies in an automated way

generate_wheels uses $WHEELHOUSE to generate and store the wheels for
requirements.pip and requirements-testing.pip (if it exists).

pip_install_requirements.sh installs requirements.pip from them if
possible (if not, then it fetches them from pypi) or, if passed the
--testing flag, it installs requirements-testing.pip.

Related: #7327

- update pip
- install base reqs

this is part of a process to make the setup of the development mode less
troublesome. from now on, setting up a virtualenv in pure development
mode will be as easy as telling pip to just install the external
dependencies::

  pip install -r pkg/requirements.pip

and traversing all the leap repos for the needed leap dependencies doing::

  python setup.py develop

- Related: #7288