Query image by external interface

Some TOSCA parameters have to be fulfilled by finding the correct object in OpenStack. For instance, if the TOSCA template specifies a value for OS, we have to find the available image with the matching OS value. At the Atlanta Summit, there was discussion on storing the metadata in Glance and querying Graffiti for the artifacts. Since this support is not well defined yet, this look-up is currently hard-coded in the translator, for instance in tosca_compute.py. To be ready for the external interface, the hard-coded look-up should be replaced with calls to a separate class that would implement the look-up.