designate-manage pool update use --dry_run raise error

[ZhouHeng]

in docs [1] provides `How To Configure Multiple Pools`.it mentions the use of `designate-manage pool update --file pools.yml --dry_run`, parameter --dry_run acts as a switch. but now `designate-manage pool update`'s help is

usage: designate pool update [-h] [--file FILE] [--delete DELETE]
                             [--dry_run DRY_RUN]

optional arguments:
  -h, --help show this help message and exit
  --file FILE The path to the yaml file describing the pools
  --delete DELETE Any Pools not listed in the config file will be deleted.
                     WARNING: This will delete any zones left in this pool
  --dry_run DRY_RUN This will simulate what will happen when you run this
                     command
--dry_run acts as key value pair.
users do not know what is behind --dry-run. and now whatever the value is acceptable. this is unreasonable.

[1] https://docs.openstack.org/designate/latest/admin/multiple-pools.html

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/685887
Committed: https://git.openstack.org/cgit/openstack/designate/commit/?id=d708ff588b0ec1c341ec81e32f7f425d95e4833d
Submitter: Zuul
Branch: master

commit d708ff588b0ec1c341ec81e32f7f425d95e4833d
Author: zhouhenglc <email address hidden>
Date: Tue Oct 1 15:43:10 2019 +0800

    designate-manage pool update --dry-run not should pass argument

    --dry-run and --delete should be switchs.
    now --dry_run acts as key value pair. users do not know what is behind
    --dry-run. and now whatever the value is acceptable. this is unreasonable.

    Closes-bug: #1847393

    Change-Id: Ia6ab93395deb95173e443f9d3ea77dd52edfcf2c