charmhelpers.fetch.python.pip_install fails to install libraries when targeting a virtual environment depending on PYTHONPATH

[jguedez]

In certain cases when installing packages using pip_install with a PYTHONPATH environment variable set it can fail to install the package. An example of that is when using a charm with the operator framework, which already has a venv. If that venv already has a library installed in it (e.g pyyaml), invoking pip_install using a virtual environment will fail to install it. Pip will consider the library to be installed already since it can find it in the PYTHONPATH (even though it's actually not installed in the venv provided). An example of this situation with all the details can be found in [0]

In this case it would probably be best to ignore any PYTHONPATH so pip will install the requested libraries in the target virtual environment if they are not installed there already. Another option would be to accept an env parameter and pass it to the subprocess call.

It would also help to use check_output instead of check_call, at least at debug level so the logs can show what is actually happening in unexpected situations like this.

[0] https://bugs.launchpad.net/charm-juju-lint/+bug/1928337
[1] https://github.com/juju/charm-helpers/blob/master/charmhelpers/fetch/python/packages.py#L112