adding max_download to Landsat8 download operator

[maabdelghaffar]

This PR includes the following:

• adding get_published_products to geoserver_plugin to retrieve a list of published products via geoserver REST call.
• adding is_product_published to geoserver_plugin to return (True or False) if a certain product (published or not) also through geoserver REST call.
• adding download_max parameter to Landsat8DownloadOperator so that it doesn't exceed a limit of downloadable products coming from the search operator