concerning unittest & field validators

Fixes problems with plugins

- updated scripts etc
- make tests pass

- Replaced some "hidden" import with LodelContext.expose_modules calls
 - in some functions of Setting, Plugin, validator
 - in loader.py (temporary version)

In this state the tests runs (151 success), I'm able to run an instance (from debian package using slim) in MONOSITE mode and in MULTISITE mode by creating and loading a context in the loader.

Remove duplicate install templates directories (the same directories can be used by SLIM & runtest)
Addapted runtest so it can fetch the install template from the lodel2 datadir
Make lodel_admin.py runnable (add the python3 shebang + chmod +x at install-data target)
And a beautifull ASCII formating for the README

Adds a LODEL2LOGDIR to buildconf.py
Adds a nginx-conf option to slim
...

Now create an INSTANCE_NAME.sock in @localstatedir@/uwsgi_sockets
Note : the localstatedir is wrong -_- /usr/var instead of /var........

SLIM is now able to edit configuration from CLI, to start instances etc.