cassandra: new class ca_manager

Also rename cassandra-ca-mgr into cassandra-ca-manager, introduce a related
class to install it on the node hosting private.git

The CA manager will generate a CA, plus all related public/private
material and the related keystores to be used by Cassandra. The list of
certificates to generate is supplied to the manager by a yaml manifest, both
the manifest and the resulting material are stored in private.git.

Bug: T108953
Change-Id: I33800945788e5f9a81def508b9456d259ad608da