* Sunni tribesman fight off troops in western cities
* Major escalation of tension with Shi'ite-led government
* Tribal leader threatens to 'set Iraq on fire'
By Suadad al-Salhy
BAGHDAD, Jan 2 (Reuters) - Sunni Muslim fighters clashed on Thursday with Iraqi troops trying to regain control of two western cities, in a serious escalation of their confrontation with the Shi'ite-led ...