net/gscloud: update g20200504 (3705cba) to v4.0.4

• while here, move GH_TAGNAME to proper upstream's tags;
• GH_TUPLE must be created, as they removed 'vendor/' code (823d8ea);
• changelog? https://github.com/gridscale/gscloud/blob/v0.4.0/CHANGELOG.md

Sponsored by: gridscale GmbH