Make Cassandra engine support ConsistencyLevel on Django

[joaomedeiros95]

To add a default ConsistencyLevel the user just needs to add these two fields inside the Model definition:

• _cassandra_consistency_level_read
• _cassandra_consistency_level_write

from cassandra import ConsistencyLevel

class MyModel(CustomDjangoCassandraModel):

    __table_name__ = "my_model"
    _cassandra_consistency_level_read = ConsistencyLevel.ONE
    _cassandra_consistency_level_write = ConsistencyLevel.ALL