Add flavor and template validation

NC-205
Fix template/flavor validation during instance creation

NC-205
Add tests, refactor template validation, add doc

NC-205
Move template validation to the serializer

NC-205
Add flavor and template validation

NC-205
Fix template/flavor validation during instance creation

NC-205
Add tests, refactor template validation, add doc

NC-205
Move template validation to the serializer

NC-205
Fix instance security groups tests

NC-205
Merge branch 'feature/nc-205' of code.opennodecloud.com:nodeconductor/nodeconductor into feature/nc-205

Conflicts:
	nodeconductor/iaas/serializers.py
	nodeconductor/iaas/tests/unittests/test_serializers.py
Merge branch 'feature/nc-205' into 'develop'

Feature/nc 205