Remove old backups with freezer-scheduler doesn't work

Steps to reproduce the error.
1. Create a new job which contains 'admin action', job file content as below:
{
    "job_actions":
        [
        {"freezer_action": {
        "action" : "admin",
        "remove_older_than":1,
        "container" : "volumebackupcontainer1",
        "backup_name" : "job_protect_vol_cinder"
        }}],
        "description":"job_protect_vol_cinder"
    }

2. Wait until the job being scheduled.
3. The job will fail withe error info:
2017-04-10 22:09:14.090 30697 INFO freezer.job [-] Validating args for the admin job.
2017-04-10 22:09:14.091 30697 INFO freezer.main [-] Job execution Started at: 2017-04-10 22:09:14
2017-04-10 22:09:14.091 30697 ERROR freezer.main [-] unsupported type for timedelta days component: str
2017-04-10 22:09:14.091 30697 ERROR freezer.main Traceback (most recent call last):
2017-04-10 22:09:14.091 30697 ERROR freezer.main File "/opt/stack/freezer/freezer/main.py", line 217, in main
2017-04-10 22:09:14.091 30697 ERROR freezer.main freezer_main(backup_args)
2017-04-10 22:09:14.091 30697 ERROR freezer.main File "/opt/stack/freezer/freezer/main.py", line 114, in freezer_main
2017-04-10 22:09:14.091 30697 ERROR freezer.main return run_job(backup_args, storage)
2017-04-10 22:09:14.091 30697 ERROR freezer.main File "/opt/stack/freezer/freezer/main.py", line 127, in run_job
2017-04-10 22:09:14.091 30697 ERROR freezer.main response = freezer_job.execute()
2017-04-10 22:09:14.091 30697 ERROR freezer.main File "/opt/stack/freezer/freezer/job.py", line 418, in execute
2017-04-10 22:09:14.091 30697 ERROR freezer.main datetime.timedelta(days=self.conf.remove_older_than)
2017-04-10 22:09:14.091 30697 ERROR freezer.main TypeError: unsupported type for timedelta days component: str
2017-04-10 22:09:14.091 30697 ERROR freezer.main
2017-04-10 22:09:14.091 30697 CRITICAL freezer.main [-] End freezer agent process unsuccessfully
2017-04-10 22:09:14.092 30697 CRITICAL freezer.main [-] Critical Error: unsupported type for timedelta days component: str

This bug should be fixed.