[Buildbot] #3521: InheritBuildParameter and WorkerChoiceParameter API are using status api

[Buildbot trac]

#3521: InheritBuildParameter and WorkerChoiceParameter API are using status api
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Reporter:  tardyp     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  0.9.+
 Version:  master     |   Keywords:
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 InheritBuildParameter and WorkerChoiceParameter should be reworked in
 order to allow user to use data api.

 getChoices function of base class ChoiceStringParameter should be reworked
 to allow return via defer.

 and ChoiceStringParameter, should actually call getChoices anyway to
 generate its values.

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Ticket URL: <http://trac.buildbot.net/ticket/3521>
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