Mention gnt-cluster upgrade in UPGRADE
Mention gnt-cluster upgrade in UPGRADE

Ganeti 2.11 is the first version of Ganeti reachable (from 2.10) with
gnt-cluster upgrade. Hence suggest this method in the UPGRADE notes.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>

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