Fix a python 2.6.5 compatibility
Fix a python 2.6.5 compatibility

The upcoming python 2.6.5 release has a change that makes delattr(obj,
attr) fail for slots-enabled objects if the attr is not already set.

To prevent against this, we only run the delattr if the attribute is
already set.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

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