Update for latest lambda-proxy version

[vincentsarago]

There was a change recently on lambda-proxy (used byt the tiler) which will make the tiler fail here

ghw2019_cog-pipeline/tiler-deployment/tiler/api.py

Lines 306 to 319 in 274566f

	host = request["headers"].get(
	"X-Forwarded-Host", request["headers"].get("Host", "")
	)
	# Check for API gateway stage
	if ".execute-api." in host and ".amazonaws.com" in host:
	stage = request["requestContext"].get("stage", "")
	host = f"{host}/{stage}"
	scheme = "http" if host.startswith("127.0.0.1") else "https"
	qs = [f"{k}={v}" for k, v in kwargs.items()]
	qs = "&".join(qs)
	tile_url = f"{scheme}://{host}/tiles/{{z}}/{{x}}/{{y}}.{tile_format}?url={url}"

Instead of the whole block to retrieve the host url, you can now use APP.host. see implementation in https://github.com/vincentsarago/lambda-tiler/blob/master/lambda_tiler/handler.py#L114-L128

cc @abarciauskas-bgse