If the mgr dashboard doesn't restart fast enough then the inject
dashboard task will fail with a HTTP error 400.

Error EINVAL: Traceback (most recent call last):
  File "/usr/share/ceph/mgr/mgr_module.py", line 914, in _handle_command
    return self.handle_command(inbuf, cmd)
  File "/usr/share/ceph/mgr/dashboard/module.py", line 450, in handle_command
    push_local_dashboards()
  File "/usr/share/ceph/mgr/dashboard/grafana.py", line 132, in push_local_dashboards
    retry()
  File "/usr/share/ceph/mgr/dashboard/grafana.py", line 89, in call
    result = self.func(*self.args, **self.kwargs)
  File "/usr/share/ceph/mgr/dashboard/grafana.py", line 127, in push
    grafana.push_dashboard(body)
  File "/usr/share/ceph/mgr/dashboard/grafana.py", line 54, in push_dashboard
    response.raise_for_status()
  File "/usr/lib/python2.7/site-packages/requests/models.py", line 834, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
HTTPError: 400 Client Error: Bad Request

Instead we can trigger this task before the module restart.

Closes: #4565

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>
(cherry picked from commit 3f6ff240)