Green Day's front man Billie Joe Armstrong is entering rehab for substance abuse ... a rep for the band tells TMZ.   The news comes just two days after Armstrong went ballistic in Vegas during a performance.  He was videotaped screaming obscenities, slamming Justin Bieber and violently smashing his guitar on-stage.  The band's rep tells TMZ,  "Green day wants to apologize to those they offended at the Festival in Las Vegas.