[Python] Adds allOf/oneOf/anyOf composed models (#4446)

* Adds composed model support for allOf oneOf anyOf

* Moves discriminator into discriminator() class method, adds test_deserialize_lizard, adds setting discriminator using allOf schema

samples/client/petstore/python-experimental/test_python2.sh

@@ -8,16 +8,18 @@ DEACTIVE=false
 
 export LC_ALL=en_US.UTF-8
 export LANG=en_US.UTF-8
+PYTHONPATH="$(which python)"
 
 ### set virtualenv
 if [ -z "$VIRTUAL_ENV" ]; then
-		virtualenv $VENV --no-site-packages --always-copy
+		virtualenv $VENV --python=$PYTHONPATH --no-site-packages --always-copy
 		source $VENV/bin/activate
     DEACTIVE=true
 fi
 
 ### install dependencies
 pip install -r $REQUIREMENTS_FILE | tee -a $REQUIREMENTS_OUT
+pip install -r test-requirements.txt
 python setup.py develop
 
 ### run tests