timeout for build steps

[faryon93]

The sackci server should kill too long running build steps. Therefore a per-step timeout (in seconds) should be configurable in the Pipelinefile.

Example Pipelinefile:
... {"stage": "Build Binary", "image":"gcc", "steps": [ {"cmd": "gcc -Wall -o test test.cpp", "timeout": 120} ]} ...

If no timeout is configured for a build step, a fallback value should be configurable in the projects defintition.