Update DockerContainers Endpoint capability corresponding to a service name

[adidanes]

Description

When a DockerContainer defines a tosca.Capabilities.Endpoint, the alien4cloud-kubernetes-plugin will generates a K8S Service resource for it. The service name is also generated by the plugin.
This issues requests the following feature to be implemented:

• if the Endpoint defines a property having the name service_name, its value should be overriden by the generated service name (this name is also set to the metadata.name of the generated K8S service spec)

What usage for this feature

This feature can be used to set the value of the an ENV_{INPUT_NAME} variable allowing to configure a DockerContainer that defines a requirement on a target DockerContainer that has an Endpoint capability defining a service_name property.
Example:
inputs: ENV_YORC_HOST: { get_attribute: [TARGET, enpoint_name, service_name] }

where enpoint_name corresponds to a capability of type tosac.capabilities.Endpoint requested on the target:
requirements: - endpoint_name: capability: ... <derived from tosac.capabilities.Endpoint>

If the value of the service_name capability property is set to the K8S service name as required by this feature, the ENV_ input variable can be set to the actual K8S service name.