Add DOCKER_EXPERIMENTAL environment variable

The DOCKER_EXPERIMENTAL environment variable drives the activation of
the 'experimental' build tag.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

utils/experimental.go

@@ -0,0 +1,7 @@
+// +build experimental
+
+package utils
+
+func ExperimentalBuild() bool {
+	return true
+}

utils/stubs.go

@@ -0,0 +1,7 @@
+// +build !experimental
+
+package utils
+
+func ExperimentalBuild() bool {
+	return false
+}