Add boolean option to distribute the private key which is
created by the cli-enable-ssh-admin.yaml playbook and update
the tripleo_create_admin role to distribute the private key
when it is true.
This option defaults to false as we normally don't want to
do this. However, cephadm needs a private key on all nodes
with the OS::TripleO::Services::CephMgr service in order to
manage a Ceph cluster. This option will likely only be used
for the ceph-admin user which is similar to but not the same
as the tripleo-admin user.
Also, remove old reference to Mistral in task name.
Implements: blueprint tripleo-ceph
Change-Id: I69c74c1869aa0f54c1695fd53098df7e78f64247