Add forgotten unittest changes for instance_os_add
Add forgotten unittest changes for instance_os_add

The previous patch which fixed disk parameters didn't adapt the
unittests so it lead to failing QA.

Signed-off-by: René Nussbaumer <bc755ac45a0e799d1d490cba76febc56d80cc5fc@google.com>
Reviewed-by: Agata Murawska <4c8886dbe342c6966e25105100cfee09458faa93@google.com>

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