Update rook/cassandra Docker tag to v1.7.0

This MR contains the following updates:

Package	Type	Update	Change
rook/cassandra	ironbank-docker	minor	v1.6.8 -> v1.7.0
rook/cassandra		minor	v1.6.8 -> v1.7.0
rook/cassandra	stage	minor	v1.6.8 -> v1.7.0

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