cherry-pick fix-1388055
cherry-pick fix-1387782
cherry-pick fix-1388747
plainbox: increment version to 0.15.1
"automatic merge of lp:+branch/checkbox/release/ by tarmac [r=sylvain-pineau][bug=1331302,1341769,1347120,1383447,1387782,1388055,1388747][author=checkbox-dev]"