By Gavin Jones ROME (Reuters) - European anti-establishment parties of all political colours are rallying behind Greek Prime Minister Alexis Tsipras in his stand-off with creditors, which threatens to force Athens out of the euro zone and undermine the single currency. Parties with virtually nothing in common except a rejection of EU-imposed austerity are hoping public sympathy for Greece and its radical leftist leader will translate into a backlash against traditional parties and a boost in their own support. ...