engine.Env.Expand applies shell-style variable expansion on an input string

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

Conflicts:
	engine/env.go
	engine/env_test.go
engine.Env.Add: append multiple values for the same key

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)