Abstract builder and implement server-side dockerfile builder

This patch creates interfaces in builder/ for building Docker images.
It is a first step in a series of patches to remove the daemon
dependency on builder and later allow a client-side Dockerfile builder
as well as potential builder plugins.

It is needed because we cannot remove the /build API endpoint, so we
need to keep the server-side Dockerfile builder, but we also want to
reuse the same Dockerfile parser and evaluator for both server-side and
client-side.

builder/dockerfile/ and api/server/builder.go contain implementations
of those interfaces as a refactoring of the current code.

Signed-off-by: Tibor Vass <tibor@docker.com>

integration-cli/docker_api_build_test.go

@@ -39,7 +39,7 @@ func (s *DockerSuite) TestBuildApiDockerfilePath(c *check.C) {
 		c.Fatal(err)
 	}
 
-	if !strings.Contains(string(out), "must be within the build context") {
+	if !strings.Contains(string(out), "Forbidden path outside the build context") {
 		c.Fatalf("Didn't complain about leaving build context: %s", out)
 	}
 }