ADEN (Reuters) - Yemeni government forces have driven a group of Islamist insurgents linked to al Qaeda from their mountain retreat in the country's south after killing 21 militants during two days of fighting, a military source said on Saturday. The army and pro-government militias battled militants on Thursday and Saturday near the town of Shuqra in Abyan province, an impoverished, rugged region of southern Yemen where tribal law holds sway and armed Islamists have a strong presence. Two soldiers and five militia members were also killed, the source said. ...