By Gram Slattery ABU DHABI, June 24 (Reuters) - U.S. Secretary of State Marco Rubio begins a Middle East tour in earnest on Wednesday, seeking to reassure Gulf allies who view concessions in President Donald Trump’s Iran deal that include a proposed $300 billion fund as too generous to a regional foe. Arriving in Abu Dhabi late on Tuesday for a three-day Gulf trip, Rubio is undertaking his first high-level diplomatic mission on the agreement reached last week to end the four-month-old U.S.-Israeli war with Iran.