revise submit endpoint #60

While writing tests for #36 I noticed a few possible improvements regarding the submit endpoint:

Ports

• The container object has a list of ports objects. The mounts object contains a port object. I suggest that the container has a list of port objects. So we do not need the ports object.
• The fields host and container should be marked as NotEmpty and NotNull

Mounts

• The container object has a list of mounts objects. The mounts object contains a mount object. I suggest that the container has a list of mount objects. So we do not need the mounts object.

Mount

• Modi: At the moment the rest endpoint accepts any string. We should restrict it to the allowed values (rw and ro)
• all fields should be marked as NotNull

• host path is allowed to be null since, a volume can be specified by using container

Additional Points:

• add container cmd field as mentioned by @jkrue in cmd attribute of task is annotated as "not empty" #56 .
• clarify whether stderr and stdout are path or not as mentioned in revise stateGet rest endpoint #58