Fix backend.{Start,Stop}Master
Fix backend.{Start,Stop}Master

Commit c26a6bd21c17641f718369caed88ae16947fa774 changed GetMasterInfo
not to return a tuple anymore, but didn't update its two callers in
backend.py, which were trying to extract the values from the second
tuple element. This causes a stack trace in node-daemon.log.

Signed-off-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

bitemyapp/ganeti,dimara/ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,yiannist/ganeti,apyrgio/ganeti,ganeti/ganeti,mbakke/ganeti,grnet/snf-ganeti,andir/ganeti,apyrgio/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,leshchevds/ganeti,leshchevds/ganeti,ganeti/ganeti,andir/ganeti,apyrgio/snf-ganeti,yiannist/ganeti,leshchevds/ganeti,yiannist/ganeti,ganeti/ganeti,bitemyapp/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,dimara/ganeti,grnet/snf-ganeti