update elevation command with full functionality

[nilsnolde]

https://github.com/valhalla/valhalla/pull/3318 added much more functionality to the elevation downloader which we should make use of (while trying to stay backwards-compatible):

• download only tiles covering the graph. that's a bit tricky in this automagic image, since we first have to build the graph, then download elevation files based on that, then start the graph build again before the elevation builder stage. somewhat makes me want to solve https://github.com/valhalla/valhalla/issues/3311 first
• download tiles per geojson files, rather optional I'd say

[nilsnolde]

actually only building tiles covering the graph is more than enough, screw "backwards-compatibility" (doubt anyone's gonna notice), this is so much more convenient than figuring out the right bbox beforehand..