Fix system test vcenter_vmdk

There is ostf test 'Launch instance, create snapshot, launch instance from
snapshot' which never works in Fuel 5.x with vCenter hypervisor due to 
timeout problem. And in system test vcenter_vmdk it was marked as 
'should fail'.

Now problem with timeouts solved and mark 'should fail' must be canceled.

Change-Id: I2ed6c9359fd58ab37133bab133569273c9084a54
Closes-bug: 1400287
Merge "Fix system test vcenter_vmdk"