Allow users to use a virtualenv as own python

[ktf]

Notice that it will still try to install our own provided
packages so that we have a minimum working environment.

[ktf]

@aalkin @singiamtel @pzhristov @ddobrigk @adriansev This allows alibuild to reuse an already existing virtualenv which is active at the moment of the build.

[singiamtel]

Would this cause trouble with the other packages that try to handle venvs? Namely xjalienfs (just saw the full diff) and machinelearninghep

[ktf]

In principle no, they will just install as usual and think that python comes from the virtualenv. It's however a good idea to add similar support also for anything which tries to install python packages. Once https://github.com/alisw/alibuild/pull/875/files is merged, we can even automate the rebuilds if one of the tools changes.