improve: continue allocating if remain tasks is more than job's minMember

[lowang-bh]

Consider this scene:
part of pods of a job is not schedulable, and left pods are schedulable, and the number can meet up the condition of min member.

So this pr fix this issue: if allocated pod less than minMember and tasks are not used up, will continue on, untill the number of remained tasks can not meet up the condition of min member

commit 1: continue allocating and testcases about the feature change
commit 2: use string type to replace the taskID type, which indicate a role spec of each task in a job

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[lowang-bh]

/assign @Monokaix @hwdef @william-wang

[hwdef]

It looks like this PR does two things. But use one commit

[lowang-bh]

It looks like this PR does two things. But use one commit

Have splited them into two commits.

[lowang-bh]

@Monokaix @hwdef @wangyang0616 @william-wang please help to review it. thanks.