A Trial to Evaluate the Safety and Efficacy of 2 Treatment Regimens of Aztreonam 75mg Powder and Solvent for Nebulizer Solution/ Aztreonam for Inhalation Solution (AZLI) in Pediatric Subjects with Cystic Fibrosis (CF) and New Onset Respiratory Tract Pseudomonas aeruginosa (PA) Infection/ Colonization