Fix the downgrade function of cfgupgrade
Fix the downgrade function of cfgupgrade

The downgrade was not correctly removing some of the UUIDs (namely, those
of disks and NICs).

Fixes Issue 510.

Signed-off-by: Michele Tartara <05f7077a2c6cbbdc4af4ba42e3ef89a0df846179@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

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