Add condition on dashboard installer phase

Add condition on dashboard installer phase

Even if dashboard feature is disabled then the installer status will
still report dashboard, grafana and node-exporter roles timing.

INSTALLER STATUS **********************************
Install Ceph Monitor           : Complete (0:01:21)
Install Ceph Manager           : Complete (0:00:49)
Install Ceph Dashboard         : Complete (0:00:00)
Install Ceph Grafana           : Complete (0:00:02)

When need to set the dashboard_enabled condition on those installer
phase pre/post tasks.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>