I think it was a perfect epilogue to Age of Ultron while hinting at Civil War to come, so I think making it the end of Phase Two works.
That being said, Phases were something invented so the public wouldn't just think "Iron Man, Iron Man sequel" but think of the connected universe as a whole. Given that, they've somewhat outlived their usefulness. Phase two doesn't really have much of a connected theme - the Infinity Stones may be the closest - so it's hard to say whether Ant-Man was more part of this phase or not.