Feature - Configurable ConsistencyLevel

[hhandoko]

Summary

Related tickets: #20 , #46

ConsistencyLevel is now configurable via cassandra.migration.keyspace.consistency system property (in addition to setting KeyspaceConfig.consistency property).

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Pull Request (PR) Checklist

Documentation

• [x] Documentation in README.md or Wiki updated
• [x] Update Release Notes if applicable -- collaborator-access only

Code Review

• [x] Self code review -- take another pass through the changes yourself
• [x] Completed all relevant TODOs, or call them out in the PR comments

Tests

• [x] All tests passes