Document iallocator change (alloc_policy)
Document iallocator change (alloc_policy)

Signed-off-by: Balazs Lecz <024c580adbe3324c61e798a30c60cf94ec02e89f@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

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