Add dummy version of batch_write to cassandra impl

This is very basic realization of batch_write operation.

Change-Id: I642382b3d4bc91b94336ff5c19c175ef242b4f11
Implements: blueprint cassandra-batch-operations
Add batch_write_item API v1 method

Add basic realization of batch_write_item API method.

Change-Id: Ic21246c3fd06a1957dd89df86422702212e06c45
Implements: blueprint os-restful-api-batch-write
Merge "Add dummy version of batch_write to cassandra impl"
Merge "Add batch_write_item API v1 method"