support for multiple clusters in dashboard

This is to allow the users to select the cluster
which the data needs to be displayed.

dashboard/osd-viz/graph pages will be refreshed
according to the selection of the cluster.

Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>
Merge pull request #58 from kmkanagaraj/multi-cluster-support

support for multiple clusters in dashboard