ability to rotate the default "cassandra/cassandra" root user

[Aaron Gershman]

but will still need an ability to login using ./cqlsh ... in tserver post-start script. So perhaps an ability to pass a list of "old passwords" to attempt trying to sign-in with

part of #34

see: https://github.com/orange-cloudfoundry/cassandra-boshrelease/blob/f8e69356ba216cfa5eda1b2197d954970ce43c13/jobs/cassandra/templates/bin/post-deploy.sh#L98