modifying original PR [0] by cristoph to account for the recent
vendoring of l2db code, which means we no longer depend on u1db/dirspec.

I expect the whole mess about the venv setup to be further simplified
pretty soon, since we are going to merge most of the leap.* packages
into a couple of repos.

[0] https://github.com/leapcode/soledad/pull/327

From this moment on, we embed a fork of u1db called l2db.

Old versions of pip do not accept the --trusted-host option and will complain
when trying to upgrade pip from wheel. To fix that we upgrade pip from usual
location instead of doing it from wheel.

--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