“Alien: Covenant,” Rated R, 123 minutes. Two and a half stars. Ah, the siren song of John Denver. Who among us can resist it? Certainly, not the crew of the Covenant, a vessel powered by a golden sail cruising through space with 2,000 “colonists” in hyper sleep and years to go until they reach their destination. But when a shock wave from a solar flare jostles the crew awake, they soon begin hearing a faint transmission of “Take Me Home, Country Roads” emanating from a curiously Earth-like planet.