By Haseeb Alwazir and Nidal al-MughrabiGAZA/CAIRO, July 23 (Reuters) - In the sprawling tent camps of southern Gaza, a disposable cigarette lighter has become an unlikely luxury item, as essential to daily survival as food, water and fuel.For displaced mother-of-four Nisreen Abu Saada, lighting a cooking fire can be a daily struggle in a territory where gas remains scarce and families rely on burning cardboard, plastic and scraps of wood to prepare meals."The tent is useless without a lighter," she said.