software/grafana: promise to check influxdb is usable

This should fix this random failure with tests:

    ======================================================================
    FAIL: test_influxdb_api (test.TestInfluxDb)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "parts/slapos-repository/software/grafana/test/test.py", line 126, in test_influxdb_api
        [connection_params['influxdb-database']], result['series'][0]['values'])
    AssertionError: ['telegraf'] not found in [[u'_internal']]