Add "keep" option to system, recipe and/or recipeset, so the machine does not get recycled when the job is done, but current user is kept, until it is returned manually.

== Rationale ==

- sometimes a problem occurs during test execution, which needs developer attention,
- and as this is not know in advance usually user cannot add /distribution/reservesys to recipe,
- and even with reservesys in the recipe, it would not work when recipe gets aborted.