dashboard: allow remote TLS cert/key copy

dashboard: allow remote TLS cert/key copy

When using TLS on the ceph dashboard or grafana services, we can provide
the TLS certificate and key.
Those files should be present on the ansible controller and they will be
copyied to the right node(s).
In some situation, the TLS certificate and key could be already present
on the target node and not on the ansible controller.
For this scenario, we just need to copy the files locally (on each remote
host).

This patch adds the dashboard_tls_external variable (with default to
false) to allow users to achieve this scenario when configuring this
variable to true.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1860815
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>