Re-Introduce the "Entry-Service-Template" Field in CSAR

In cases in which a CSAR packages multiple <ServiceTemplate> elements the existence of a Entry-Service-Template field allows an efficient processing of the <ServiceTemplate> elements: By being pointed to the main service template the contained imports can immediately be resolved. Without the Entry-Service-Template field, all of the N contained <ServiceTemplate> elements must be inspected and analyzed, some sort of dependency graph must be build, and the "main service template" (i.e. the entry to the set of N service templates) must be identified. For large numbers of N, this is inefficient. Thus, for CSARs that contain <ServiceTemplate> elements it should be possible to set the value of Entry-Service-Template field. For CSARs that do not include <ServiceTemplate> element setting a value for the Entry-Service-Template field is not meaningful. As a consequence, the Entry-Service-Template field should be made OPTIONAL.