(Reuters) - It's been the stormiest year for Italian bond markets since the 2011 euro zone crisis, but as 2018 ends, investors who fled mid-year are returning and their worst fears -- euro exit, junk ratings and reckless spending -- have proved unfounded.Often described as bond vigilantes for the way they impose fiscal discipline on profligate governments, debt investors punished Italy hard for proposing a draft budget deficit three times higher than it had pledged. Sovereign borrowing costs..