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

Expose some statistics from the simple scheduler, so that the status page does not break when it is enabled.

Make the Zoe master more resistant to Swarm being offline

See merge request !23

Make the Zoe master more resistant to Swarm being offline

See merge request !22

Try to make sure all timestamps in Zoe are processed in UTC

See merge request !21

Implement two feature requests

See merge request !20

Fix exception for recovered executions after restart

See merge request !19

Fix bug with inverted volume definition during service creation (fixes bug #58)

See merge request !18