Implementation of JobWorker, WorkerCommandHandler, server -> worker commands

[JohannesGaessler]

Fixes https://github.com/DistributedTaskScheduling/JobAdder/issues/65 . Fixes https://github.com/DistributedTaskScheduling/JobAdder/issues/66 .

As discussed in the meeting, I'm taking over the task of implementing the worker main class and the WorkerServerCommand classes. I changed the interface of the execute method of WorkerServerCommand classes to use DockerInterface instead of JobWorker. In addition to the things implemented in https://github.com/DistributedTaskScheduling/JobAdder/pull/112 I also implemented the following things that were missing:

• Error handling in the execute methods of commands.
• Registering the worker on the server.
• Storing the worker UID provided by the server in the config file.
• Checking user permissions.

[JohannesGaessler]

I also added admin_group to WorkerConfig.

[nikolatzotchev]

This looks good to me, only one question why do you keep the get_workload class, if we talked that we are not going to implement it.

[JohannesGaessler]

Because it's an optional feature that might get implemented someday if our project sees actual use.

[JohannesGaessler]

To be honest the unit tests are just something I forgot.

[nikolatzotchev]

I think this is good, let me know if you plan to add tests for the messages, otherwise I will approve. I'm not sure they are is such a need, because the important part (docker_interface) is already tested.

[ammen99]

Seems like the Travis build also passes, I hope this issue is resolved soon.