Complete remote driver

This patch completes the work started in creating a remote driver:

- Renames the env driver to the remote driver (an alternative suggestion
  that should be more user-friendly)
- Adds support for TLS to encrypt connections with buildkitd
- Fixes outstanding review comments
- Reworks the buildx create command endpoint construction to be clearer
  and include better support for this new driver.

Signed-off-by: Justin Chadwell <me@jedevc.com>

cmd/buildx/main.go

@@ -23,8 +23,8 @@ import (
 
 	_ "github.com/docker/buildx/driver/docker"
 	_ "github.com/docker/buildx/driver/docker-container"
-	_ "github.com/docker/buildx/driver/env"
 	_ "github.com/docker/buildx/driver/kubernetes"
+	_ "github.com/docker/buildx/driver/remote"
 )
 
 func init() {