Add attribute to create Cassandra as a system user
When installing cassandra via the datastax or apache packages, the Cassandra user is always installed as a system user.  This means that the uid will be created between SYS_UID_MIN and SYS_UID_MAX as defined in `/etc/login.defs`, which helps prevent conflicts with other users created later.

This commit adds an attribute so that when `setup_user` is set to true, the `cassandra::user` recipe will create a system user.  Existing systems will be unaffected, as the `user` resource does not modify the already-created user.
Merge pull request #146 from DataDog/cassandra_system_user

Add attribute to create Cassandra as a system user