IAlloc: read group uuid from the input message
IAlloc: read group uuid from the input message

This makes the code incompatible with JSON files from Ganeti pre-2.4.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Balazs Lecz <024c580adbe3324c61e798a30c60cf94ec02e89f@google.com>

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