By Gram Slattery MANAMA, June 25 (Reuters) - U.S. Secretary of State Marco Rubio will meet with Bahrain officials on Thursday on the final leg of a trip to the Middle East where he has sought to sell the Trump administration's preliminary Iran accord to skeptical Gulf Arab allies. Rubio has acknowledged his delicate mission in pitching the peace deal to Gulf Arab leaders who fear excessive concessions will strengthen Tehran and reshape the region's security balance and oil flows.