Ensure base cassandra directory is created.

This introduces a new cassandra::default_data_directory_base parameter, used
only for default instances (not multi-instances), which is used to ensure
the base directory is created.

This is fairly ugly as it introduces one more difference between single and
multi instance (note that we already have quite a few parameters following
this pattern).

A probably better alternative would be to remove everything related to
instances from ::cassandra and let the caller manage either single or multi
instance. But that's a bit more than I am willing to risk with my current
understanding of the usage of this module.

Better proposal welcomed!

Bug: T138092
Change-Id: If69ef8dfff534d178472527c8c4fc47cffa7df10