Pull request from testing model task

The pull request includes the next tasks:
- Graph creation & deletion
- Node types creation, edition & deletion
- Relationship type creation, edition & deletion
- Node creation, edition & deletion
- Instance (engine) creation, edition & deletion
- Node media file creation, edition & deletion
- Graph clone, import & export*

* The import and export test will be made in the "testing UI" task

Some original files modified:
- engines/tests -> The entire folder has been removed. The tests for the instances are in the test.py file now.
- engines/models.py -> In the Instance class, there was a mistake in "get_gdb()" method. Specifically, we changed "self.key_cert" to "self.key_file" in the "connection_dict" values.