Extend support for custom Grafana dashboards

The current behaviour is to support supplying a single
folder of Grafana dashboards which can then be populated
into a single folder in Grafana. Some users may wish
to have sub-folders of Dashboards, and load these into
separate dashboard folders in Grafana via a custom
provisioning file. For example, a user may have a
sub-folder of Ceph dashboards that they wish to keep
separate from OpenStack dashboards. This patch supports
sub-folders whilst not affecting the original mechanism.

Trivial-Fix

Change-Id: I9cd289a1ea79f00cee4d2ef30cbb508ac73f9767