Support for AWS Keyspaces with SIGV4 plugin for Cassandra driver

[bhoudu]

It is possible to connect to AWS Keyspaces (managed Cassandra) with temporary credentials thanks to SIGV4 cassandra plugin (https://github.com/aws/aws-sigv4-auth-cassandra-python-driver-plugin).

As cassandra-migrate uses the python cassandra driver (fully compatible with the SIGV4 plugin) it would be great to add AWS support for temporary credentials (generated from AWS tokens) so we would not be in the need to define a user / password in keyspaces (more secured).