mgr/dashboard: Add Grafana proxy

mgr/dashboard: Add Grafana proxy

Add the ability to connect the dashboard with a Grafana instance to
display graphs from Grafana incorporated into the dashboard.

Possible configuration options:

$ ceph dashboard set-grafana-api-host <host> # Default: 'localhost'
$ ceph dashboard set-grafana-api-port <port> # Default: 3000
$ ceph dashboard set-grafana-api-scheme <scheme> # Default: 'http'

The configuration can be checked using a GET request to the API endpoint
`/api/grafana/status`. It returns a JSON response in the following
format:

    {
	"available": False,
	"message": "<error message>",
    }

The message is not returned if Grafana is available.

The Grafana proxy is reachable through
`http(s)://<host>/api/grafana/proxy`.

Signed-off-by: Patrick Nawracay <pnawracay@suse.com>