--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

Once upon a time we needed to pin the PyOpenSSL version to avoid unneeded
crypto deps (see https://leap.se/code/issues/5368#note-5). Since then, jessie
was released and PyOpenSSL 0.14 is now shipped with it. We have removed that
pinning from the debian package, and it is not needed here.

soledad-common versions before 0.6.5 do not contain the fix for #6833 and thus
will not work with most recent server. That is why we have to bump this
soledad-server dependency on soledad.common.

In the past, we wanted dependency on leap.common to be optional, but now
because of the explicit use of the config path prefix and signaling, we want
to enforce dependency on leap.common.