By default guest users do not see any ZApp in the Shop. the new attribute 'guest_access' in the manifest file can be set to true to make a particular ZApp accessible.

Open enpoint links in new windows and prevent a race condition when a new execution is added to the scheduler queue

This feature is implemented only on the DockerEnging back-end.
When enabled via the new configuration option --backend-image-management, Zoe will (1) when an execution is submitted, start threads to pull the image on the nodes and (2) schedule services on the hosts that have the image already loaded.
This has two big side-effects:
1. Zoe will not let Docker automatically pull the image at run time and will refuse to start a ZApp with unavailable images
2. ZApps will always run with the latest images: a pull is always done before a run

For the Docker back-end the address that should be published for port
mapping should be taken from the config file. Docker by default publishes
on 0.0.0.0.

Prepare codebase for release

See merge request !24