* Atlantia, ACS strike deal to jointly buy Abertis * Atlantia to have control over acquisition vehicle * Italian group has option to buy stake in Cellnex * Deal follows Spanish concerns over foreign takeover (Adds boards' approval, Atlantia option on Cellnex) By Francesca Landini and Stefano Bernabei MILAN/ROME, March 14 (Reuters) - Italy's Atlantia and Spanish builder ACS agreed a joint 18 billion euro ($22 billion) bid for Abertis on Wednesday, ending a five-month bat