Update cfgupgrade tool
Update cfgupgrade tool

Create an upgrade function that adds a nodes field to each disk that is
attached to an instance. Also, add a downgrade function that removes the
nodes field and brings the config to the default state for 2.12.

Signed-off-by: Alex Pyrogiotis <6383a799dda2258eec8fa26a3ce2ec8a7d552341@grnet.gr>
Reviewed-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>

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