[VMM] Driver always requires contextualization image

[dchrzascik]

During deployment, it is always required that at least one additional image is present:

Fri Nov  2 14:47:54 2012 [VMM][D]: Message received: LOG I 7 /var/tmp/one/vmm/ovz/open_vz_data.rb:88:in `context_disk': Can't select context file: there
are 0 isos corresponding to this vm (OpenNebula::OpenVzDriver::OpenVzDriverError)

Fri Nov  2 14:47:54 2012 [VMM][D]: Message received: LOG I 7 from /var/tmp/one/vmm/ovz/open_vz_driver.rb:62:in `deploy'