--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

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

requirements-latest.pip will try to clone and install. Since it is meant
to be latest, I added a small change to specify the branch 'develop'.

With this, you can setup soledad for using locally
and running the tests with the latest head in a simpler
way

- update pip
- install base reqs, with insecure flags for dirspec and u1db

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

bump leap.common min required version, new change needed
'collect_plugins'.