mgr/dashboard: Add token authentication to Grafana proxy

mgr/dashboard: Add token authentication to Grafana proxy

Enables token authentication for the Grafana proxy as additional option
to username/password authentication. The authentication method has to be
set, too.

    $ ceph dashboard set-grafana-api-token <token>	   # default: ''
    $ ceph dashboard set-grafana-api-auth-method <method>  # default: ''

Possible values for the authentication method are 'password' and
'token'.

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